the rule that I expect to use if this were to come up in my game is that the spell uses Finnesse -3 or the unmodified craft score (possibly with an additional modification to either of these scores depending upon the spell).
This resolves the issue of a magus with craft 5 versus a magus with finesse 5. It doesn't make a character with craft 4 and finesse 5 better than a character with craft 4 and finesse 1.
This is just what I was looking for, thanks!
When 02 skills apply to a situation ,
i get the XP total for both skills and average them.
Use this result to work out the closest Score lower than this value.
Isn't this normal? I didn't think defence mastery caused parma to double too?
I think most of us agree, but that the RAW isn't clear enough on the issue is perhaps the reason.
The rules IIRC (serf's parma) say "doubles the magic resistance". Others have claimed that this is ambiguous. I don't think that it is (thus the house rule).
OK, I'll stick to agreeing with the house rule, which I think that most of us do, and that I believe many would prefer a errata in that direction.
Over at Project: Redcap I've started a page specifically for links to pages with house rules, but no one (other than the Redcap maintainer) has submitted any links.
If anyone has house rules posted to the Web, please consider submitting a link to Project: Redcap to share your ideas with the community.
But from what I've read of ArM5 so far, there's no particular imbalance caused by reintroducing a Certamen ability and allowing it to be puissant (other than to reduce the enormous benefit many Tremere magi will get from using magical focus).
Or you can call it a knack that gives +2 to certamen abilities (whatever the Te+Fo combo) and be done with it. Rules are there for our convenience. if something inconveniences you, change it. Easy 
Cheers,
Xavi
If reading books are the best way of advancement I think the +3 bonus of book reader virtue is too high. If I get 12 as the average book quality the virtue increase the advancement by 25%.
I find it too many for a minor virtue. I would reduce its bonus to +1.
I think it would be an interesting campaign if it would be impossible to create/improve things permanently with Creo rituals. No gold making, stat improving, and no healing of course.
Who needs creo and vis when you have rego and skilled craftsmen? 
Also, ring duration can substitute for vis in a lot of cases. Widely used in our last campaign.
Cheers,
Xavi
Would you still allow CrCo spells to suspend healing?
Could you still give a boost (from +01 to +18 ) to Recovery rolls?
You will likely end up with the situation that characters can be laid up for a Season (or more) while they heal.
For a magus this probably means no Lab work and/or any study totals halved.
(Activities while injured , page 178)
Not saying this is a bad idea , as it fits better with a Low Fantasy campaign ,
something like the Pendragon Game.
Can't do much with Rego if you don't also have the raw materials, in sufficient quantity.
Hey if you like it go for it. I would miss it though. Especially those effects are the kind of things that give vis its famed covetness. In any instance it would probably make for a more low fantasy version of Mythic Europe.
Where one metaplot of my saga is the hunt for a breakthrough, and the stories that spins from it, to be enabled to do some of those rituals with less or no vis - maybe in the version you are contemplating you could make a quest toward actually being able to do it at all.
Touché! 
Ravenscroft and Furion Transsanus
Low magic? What do you speak about?
Deleting the instant healing option would put this campaign to low level magic? Come on, be serious!
Other ideas:
I would not allow both affinity and puissant art to a single art. Choose one.
And I would set to magical foci a constant +6 bonus.
Reason: 5th edition supports overspecialized magi too much.
Then most people will likely take Affinity , as the XP saving is substantial.
You simply ditch a +03 bonus , making this virtue much less useful for low level casters.
(which may be what you want)
Will you also do the same for Abilities?
Low Fantasy , does not mean low level magic.
Taking out fast healing (of any kind) so that people , especially Magi are "more mortal" (i would read "Paranoid about leaving Labs , ever!") ,
is what i call a Low Fantasy option.
I was in a game some years ago where the SG limited the availability of vis with house rules. He was going for a bit of a lower power level, and of course in 4th vis was mighty. The rules IIRC went something like this:
Vis took time to harvest ... 10 - (vim form bonus) in days per pawn, and only Magi or other Gifted people could do it. This had the game effect of taking time away from lab activity, and raising the economic 'value' of vis. This was also to encourage more people to take Vim, as there were the twighlight implications with having hig vim in 4th. That no longer applies in 5th.
Vis extraction was limited by the aura strength. +1 aura :1 pawn per season ratio. This had the great story effect of having us searching for good auras to build vis extraction labs in. This rule was not a probelm IC because vis was considered such a precious commodity that it was well worth extracting it.
Study from vis was more productive (I think he doubled the XP from it) so it caused a greater demand for vis as a study tool. There was already a limited supply - which became scarcer still. Reinforcing the 'vis is valuable idea'.
Mystical animals, things with might, etc. did not produce vis on death. It was too easy to get vis this way to allow it in his setting. IIRC he instead gave the resultant materials really incredible form/effect bonuses to make up for this.
There were a few other things he did like change 4th ed alchemy somehow. Covenant creation and all relative vis values were adjusted (i.e. 1 pawn of vis = 100 silver pounds in his setting ... if it could be baught at all). I even remember us being excited about finding some items so we could cannibalize them for vis (someone could do that somehow - wasn't me so I didn't know exactly). Anyway, he really looked at Vis avaialbility, use, supply, demand, etc. and reworked the 'vis economy'. It really changed the feel of the game, and our veriditous player was given some means of using less vis - making the Veriditous magic very very useful. People were beating a path to her door so she could make items at reduced cost ... and she still made a commanding profit.
If one wanted a 'low magic' setting for Ars it could work in 5th. The nice thing is that Magi got a little cooler, becuase spells were the way to do things and what would have been solved by a ritual before was done with smaller, shorter duration formulaics. The Magi who did these things had more 'repeat customers' or were considered very useful to the covenant/tribunal/etc. becuase of their spells.
I find this odd. The bonus for Apt Student is higher (and Teaching is MUCH better than books for advancement); would you change that? Book Learner doesn't change the number of times you can read a Tractatus, or when you need to stop studying from a Summa. Have you seen this abused in some way I'm not thinking of?