It's a bit unclear to me how the flaw Chaotic Magic works. Let's say you're aiming for a spell at level 20. Will the flaw kick in whenever I roll 19 or 21? Or will it kick in if I roll 0-21?
And to make this more interesting, how would it work with Life-linked Spontanous Magic? 
Chaotic Magic means that if you exceed or miss your declared spell level (made before you cast) by MORE than 1 point (i.e. 18 or less or 22 or more for a declared target level of 20) then and only then do the chaotic results kick in. The spell still works (its not a botch) but it has strange side effects, is how I would understand it.
An example might be instantly making the ground under the caster's feet turn into peat bog so he sinks up to his waist when casting Teeth of the Earth Mother and missing the mark, or worse depending on the SG.
Linking this flaw to Life Linked Spontaneous Magic might seem interesting but frankly it appears to be too much a matter of sheer luck for spontaneous spells (which, let's face it are any young magus' bread and butter).
Considering that rolling less than the declared target level by too much would not only trigger the chaotic stage show (or for any scheming SG perhaps even a chaotic backlash upon the caster) but could also lead to rapid accumulation of wounds (more so by inviting some nasty uses of the chaotic effect to boost the damage) and death.
You prepared for that kind of risk?
Ah, I won't bother with that then. 
I will rather go for Flawless Magic and Twilight Prone. He knows how to do it.. alot.
E.
If you wish to make your spont-focused magic more "interesting" but still manageable, I kindly suggest to pick Unpredictable Magic, Weird Magic, or both. Not Choatic Magic. If you really need a spont-oriented major Flaw, I instead suggest Magic Addiction or Rigid Magic.
Well, I'm not set on making it spontanous oriented, just interesting, but still manageable.
E.
Well then perhaps if you want some real brainstorming from the rest of us, might it not be useful to provide us with your rough conceptualisation for the character (House, background idea, age, etc.) so we can have more to go on?
Ah yes, I'm making a Bjornaer, Clan Maruhs magi. He has an otter as a heartbeast, and is very good at Animal and Aquam. I'm also aiming for the Dream Magic mystery (initiated through clan Maruhs). I'm looking for magic that is fluid, dreamlike, chaotic, poetic.
E.
Well first thing that springs to mind then is to consider going more the route of mystery virtues and not necessarily from the strict Bjornaer chapter. Since Clan Maruhs magi are "free spirited", and "capricious" by the HOH:MC definition, why not examine (and discuss with your SG) some Merenita virtues such as Glamour, or even Perpetuity (for interesting spell duration effect capabilities and a ready made flaw), one of the Nature mysteries of Merenita even (some seem to fit perfectly?
A perfect justification for this would be having been taught such a mystery (or mysteries) by a descendant of one of the outcast Merenita that were assimilated in House Bjornaer.
Other suggestions might be:
Dream Interpretation RoP:TD, pages 136, 138
Hermetic Dream Interpreter TMRE, pages 62-63
Dream Magic TMRE, pages 102-107
etc.
I dont have many of the other books like Societas or TMRE but if you do I would scour them using this excellent Virtue/Flaw index
atlas-games.com/pdf_storage/ArM5VFIndex.pdf
Also note that the otter is an indigenous North American animal, not European, so its already a bit of literary license using it for a heartbeast. Nevertheless, borrowing from a source on Native American animal symbolism for the associations given to the otter:
So any virtues having to do with any of these aspects would be a good idea.
Hope that helps.
About the otter, well, there are many species (animals is a big interest of mine). Otters can be found all over the world, and the one I'm using as heartbeast is the Lutra lutra, or European Otter. Then you have otters in Brazil, the Giant River Otters, Sea Otters out on the coast of California, Oriental Clawless Otters in Asia, Northern River Otters in the US. You even have otters in Africa, in the Congo.
When it comes to the virtues, I think I want to keep away from Merinita, I don't want to go to far with the concept. One idea I had was to have a gifted magus (Flawless Magic) but just underneath there are wild creativity trying to get out. So I'm trying to find a way to simulate this internal conflict. The stronger his heartbeast becomes, the more this wildness takes over.
E.
Ah I see.
Well just consider that whilst Flawless Magic is certainly a great virtue (perhaps second only in power and flexibility to that of MMF), I would question whether a nature based concept such as you seem to be aiming for really needs a more spellcraft oriented virtue? Is he going to be spending alot of time in the lab making spells?
Seems a more outdoorsy virtue set drawing on the nature of magic or the magic of nature themselves would be more in keeping with a Bjornaer, no?
Some of the possible virtues I mentioned above have nothing to do with Merenita so perhaps one of the Dream oriented ones or maybe even spirit oriented or enchanting music or somesuch? Those seem more in tune with your stated objectives.
Good luck anyways!
Chaotic magic isn't really that bad. to have things go haywire you need to be off from your projected level by more than 1.
Suppose my character's applicable casting total is 28.
I would then always shoot for fatiguing spont spells with a level of 17.
If I roll a six or seven I get level 17.
If I roll an eight or nine I get level 18
If I roll a four or five I get level 16
Six out of ten choices on the die I get the spell that I want.
If I roll a two or three I can throw in a confidence point and get the spell that I want.
If I roll a 1 followed by a 2,3,or 4 I get the spell that I want as well.
Only if I roll a zero or a one followed by something that raises the result over eight do I get the wild spell. Thats an 83% chance of getting the spell that I want.
And if I don't get the spell that I want, I didn't botch, I still got a spell off and unlike botches it isn't necessarily a bad thing.
You just need to be adept at designing spells to your target level.
Nevermind, point retracted.
Could see if your SG would let you take flawless while you are in human form* and chaotic as an otter. Possibly count them less towards your total virtues/flaws since they are not full time effects.
*would have to master each spell while in otter form if you want to use spell mastery as such
If I'm going with the Chaos-magic concept, I want Life-linked, Life-boost, Chaotic and Wierd magic. How would that work together??
E.
Youre really eager to have your character bite the dust quickly I guess. lol.
Given what I already pointed out about the inherent danger of the Life Linked Spont Magic/Chaotic Magic combo (all due respect to Erik's arguments to the contrary), adding Life Boost and a botch die-adding flaw like Weird Magic to the mix is almost guaranteed to result in a one way trip straight to the grave or Twilight, however spectacular such an exit may be for your character.
Pyrotechnics are fun to watch, less fun to BE! 
Life Linked Spont Magic !!!!
Life linked spontaineous magic allows you to add five to a casting roll for fatiguing spontaineous magic.
That means that with both chaotic magic and life boost, the only way that your chaotic would ever kick in is
- if you botch,
- if you roll exceptionally high or
- if you don't have enough fatigue or health to meet the spell
So with life linked spontaieous magic you nearly completely alleviate the penalty of chaotic magic.
No Erik, that's Life Boost youre thinking of. Life Linked Spont Magic is another virtue such as Chaotic Magic which requires you to name a target level before casting (including a number of levels of penetration if you wish, i.e. you could declare you are casting a level 10 spell at level 20 with 10 levels of Pen...) If the roll is higher than the pre-declared level you only spend 1 fatigue as with normal Spont casting. If it is lower then for every 5 points or fraction thereof you must spend an additional fatigue level.
When you run out of fatigue levels with this virtue you start to take wounds and the amount of levels by which you missed the target level count as the amount by which the damage exceeds your soak.
You may think that chaotic magic isn't that great a threat but Im afraid to say the dice don't often play by your statistical assumptions and all it would take is one spectacular failure with LLSM/CM combo and a character could be dead or gone in a flash, depending on the whims of the SG.
My understanding of life linked spontaneous magic matchs what you posted: if you don't make the spell level that you state, you end up spending additional fatigue levels and taking wounds to make up the difference.
Ergo, unless you run out of fatigue and health you meet the level that you choose. If you meet the level that you choose, then chaotic magic doesn't kick in.
What you stated previously was not what you stated just now. The description you gave above was for Life Boost, not LLSM (LLSM doesn't add +5 for every fatigue level, LB does).
As to your belief that Chaotic Magic wouldnt kick in, again not necessarily true. The CM flaw operates entirely separate from LLSM, thus if the caster missed his declared target level by 2 points or more it WOULD kick in (along with the added fatigue level for LLSM IF it was 2 or more points UNDER the declared level. Of course if he made his level exactly or off by 1 point only then yes, CM wouldn't kick in but the added fatigue for LLSM well could.
As I suggested to Ferretz above, whilst it might make for an intriguing character, the gamble taken with every spont spell cast ranks up there with the most foolhardy of Las Vegas casino addicts. All the more so since spont casting is the majority spell activity for most young magi.
If he wants to go that route, I wish him much luck! lol.