Designing a magus I'm always used to starting with a Magic Theory of 3. However if I recall correctly, Faerie Magic can substitute for Magic Theory for most House Merinita magi. Do I still need Magic Theory 3, or would 1 suffice? The character in question will steed need some Magic Theory obviously? Any thoughts on this?
cj x
Faerie Magic isn't a complete substitute for Magic Theory. Only when experimenting, when using faerie vis, or when practicing magic associated with the Faerie realm. You'll still need a MT of 3 to set up a lab that doesn't cause a penalty, or if you don't have enough Fae vis on hand, for example.
Good point! Yep, I'll keep MT 3 then! 
cj x
Very little reason for a dedicated Merinita to take Magic Theory above 3, though, unless the character wants to easily increase the amount of faerie vis she can use in a season (I believe the two scores add together in that case). And there is something to be said for spending the 30xp on Magic Theory 2 (perhaps with a specialty in setting up labs?) and Faerie Magic 2 instead of Magic Theory 3.
On the occasions that it has come up in my sagas the Merinita characters strongly favor magic theory over faerie magic simply because texts are more easily obtained (there are already MT texts in the covenant and your fellow magi will be trading, working and scheming to get more of them). The characters have always had labs in magic auras and magic theory can be used to craft spells with their faerie parameters they get with the house virtue. Things might change as the characters get older but I think that it shouldn't be at all unusual to see elder Merinita, even some who've undergone several initiations, without any skill in faerie magic. If you've already got a significant score in magic theory do you really want to invest the large chunk of XP in Faerie magic that it would take to make it as useful for you as your already existing MT score for its increased utility, or do you spend the time that it would take to accomplish that task doing something more immediately beneficial?
Its value may also depend on which mysteries the Merinita is initiating. My recollection is that the whole Folk/Story Magic path is basically useless without a score in Faerie Magic, as well as Becoming and Arcadian Travel. Also I think it figures prominently in faerie familiars...
Yes, I generally agree with both Eriks, though it's not a very useful Ability for Becoming.
Chris
I don't have my books in front of me, but doesn't Faerie Magic figure into the Becoming formula for calculating your Might Score? (Warping Score x Faerie Magic) maximum, I seem to recall, but I could be misremembering.
Yes, it does, but only until you get Faerie Might, at which point you lose your Warping Score and Faerie Magic doesn't figure in any more. That's why it's essentially irrelevant. You have to have a score of 1, but that's sufficient to do as you wish.
Chris
With a Faerie Might of around 10? Isn't that sort of weak?
Merinita pursuing Becoming in-canon explicitly commonly like to warp themselves. Where does that limit around 10 come from? Certainly not from that formula. Also, whatever the limit, that is just the limit the first time the ritual is performed. You can perform the ritual again later unhindered.
Chris
Sure, but isn't a Warping Score of more than about 6 really dangerous for a maga? It sounds to me like the Merinita with almost no Faerie Magic is trading an early Becoming ritual for a very weak Might Score, or alternatively risking a scary high Warping Score for not much benefit.
Getting 1 Warping Point at a time is not dangerous for most magi. Most laboratories are relatively secure. Add some circular wards or use MuTe or whatever to seal off the lab to virtually everyone. If you have a Faerie Magic score of 1 with a specialty in Becoming, you have a 2 as a multiplier. At the beginning of the season in which you're doing the ritual, warp yourself slowly for a few days. You only need a Cr+Vi+Sta around 15-20, depending on the Aura and how you do your rounding, to safely cast the necessary CrVi spell once per round spontaneously without tiring yourself. You can pick up 600 points in an hour. In the first hour (or longer since you probably take your time) you should be safely around a Warping Score of 16. You can easily go further if you want - you have a few days to work with.
Regardless of the point at which you stop, if you spend a second season you can reach any total your Arts allow because you got a Faerie Might score the first season, meaning you are safe from Twilight and you have no more limit on your Becoming lab totals.
Even neglecting this part, Transforming the Body can commonly be done with ease with a low Faerie Magic score and a relatively low Warping Score. And that alone makes Becoming far from worthless. The end result is that Faerie Magic really isn't too critical to Becoming. It is much more critical to other Mysteries.
Chris
Would it be apporpriate to use the Faerie Magic score instead if setting up a lab in a Faerie aura?
A hermetic lab? No.
It's not about attuning the place to the local aura,it's about knowing the bit'n'pieces you'd need and how to set things up in a practical way.
Just IMAO ofcourse.
I would be okay with that. Faerie Magic is for Merinita magae who are really good at specialized lab work, and it makes sense to me that such a maga should be able to set up her own lab. Maybe it wouldn't be usable by magi without Faerie Magic though?
I am ok with erik's approach. Fae theory of 3 would seem adequate 
IMO, no, Faerie Magic can't be used to setup a lab. It's an ability which "represents how their Magic differs from normal Hermetic magic." One cannot have Faerie Magic without knowing Magic theory, because Faerie magic is reliant up Magic Theroy. I might entertain combining the abilities to use as the limit for refining the lab, so long as the virtues/flaws installed are faerie in nature.