Some random thoughts about Parma Magica...
Parma Magica seems weak. Certainly in the face of a specialized caster, even out of gauntlet, you can have the fire slinger and his PoFs coming out at +25 penetration with little effort. The history of the Order talks about Parma being a game changer, not only for blocking the social effects of the Gift, but also because it was so darn effective. What would Bonisagus's score have been since he'd just invented it and spent so much time inventing Magic Theory and doing a lot of politicking. I doubt that Bonisagus's score would be as high as Philippus Niger's is in GoTF when he took on Trianoma and her sister , who is my guinea pig here.
So, should Parma be an Accelerated Ability?
I don't like books on Parma, I never really have as both a player and an SG, but as a player books are the only way to progress Parma with any serious effort to make it meaningful. The secret to Parma, has always been unlocking the way for you to realize that, to borrow a phrase from Raymond E. Feist, "There is no magic." You can't learn how to do that from a book, you can only learn it through practice and experience. Practice XP from this would be 5 per season. What it comes down to when in practice, and two magi are slinging spells at each other, one is believing his magic will overcome his foe's defenses, while the other guy is believing really hard for a short time that there is no magic, in order to resist.
Without books on Parma, and putting an effective gain limit on it to 5xp per season, it will take 11 seasons (discounting any virtues here) even as an Accelerated Ability of dedicated effort to raise Parma to a score of 10.
Should Parma be rolled? I know the game is Ars Magica, but Magic Resistance is either yes or no. I like an element of chance, a saving throw (don't stone me, bro!) to avoid the magic. Yes, I know it's possible on damage to soak the damage, or for the damage to botch, but what about magic that doesn't inflict damage?
Then there's the issue of scale. I mean, what do you think a magus that is old, an an Archmagus, should they have MRs that hover in the 40-50 range? Take, for example, Philippus Niger from GoTF. He has a parma of 7, specialty of Corpus, his MR are as follows. And yes, I know he's a Rego specialist so his forms are going to be lower and his MR will generally be lower, but the guy is an Archmagus and 104 in Hermetic Years.
[tableborder][tr][th]Form[/th][th]MR[/th][/tr]
[tr][td]Animal[/td][td]42[/td][/tr]
[tr][td]Aquam[/td][td]42[/td][/tr]
[tr][td]Auram[/td][td]42[/td][/tr]
[tr][td]Corpus[/td][td]55[/td][/tr]
[tr][td]Herbam[/td][td]42[/td][/tr]
[tr][td]Imaginem[/td][td]43[/td][/tr]
[tr][td]Ignem[/td][td]40[/td][/tr]
[tr][td]Mentem[/td][td]50[/td][/tr]
[tr][td]Terram[/td][td]42[/td][/tr]
[tr][td]Vim[/td][td]55[/td][/tr][/tableborder]
It's pretty easy to design a magus even a few years post gauntlet that can penetrate those scores in the 40s, granted with some effort.
Scaling him up, based on the XP spent to get to Ability score of 7 gives him an Accelerated Ability score of 16, which makes MR look like:
[tableborder][tr][th]Form[/th][th]MR[/th][/tr]
[tr][td]Animal[/td][td]87[/td][/tr]
[tr][td]Aquam[/td][td]87[/td][/tr]
[tr][td]Auram[/td][td]87[/td][/tr]
[tr][td]Corpus[/td][td]100[/td][/tr]
[tr][td]Herbam[/td][td]87[/td][/tr]
[tr][td]Imaginem[/td][td]88[/td][/tr]
[tr][td]Ignem[/td][td]85[/td][/tr]
[tr][td]Mentem[/td][td]95[/td][/tr]
[tr][td]Terram[/td][td]87[/td][/tr]
[tr][td]Vim[/td][td]100[/td][/tr][/tableborder]
I think, with the exception of some die hard combat magi, and the paranoid, most scores will hover about 10, which means it costs 55xp, 11 seasons of effort to improve defenses over a lifetime. Philippus Niger with a score of 16 will have spent 28 seasons getting his score that high. 28 seasons out of a total of 416, or 6% of his career focused exclusively on improving his defensive capabilities.