What happens if you have extended your Parma Magica, say over your apprentice, and then you engage in Certamen with a 3rd party?
Does the apprentice get to participate in the Certamen somehow? Or at least suffer whatever the Parma Caster suffers?
Come to think of it, what happens if someone wanders into the middle of Certamen?
I agree they way you use Parma in Certamen seems to me to be divorced from it's normal use. Your using the raw score not the MR it generates. So it's more like your inherent skill at resisting magic is being tested not it's current level.
I agree with maine75man - you just use your parma score, regardless of whether you have your parma up or not, and whether it's covering only you or your apprentice as well. It's not the "real" parma you are using: it's just that your ability to resist foreign magic is part of what makes your parma stronger, so that your parma score is a good indicator of how good you are at resisting hostile magic in certamen, and thus enters the formula. It's a little like rolling Perception + Ride to judge at a glance whether a horse is a purebreed or a nag - you do not get a penalty to the roll if you are Lame or Arthritic, even though you would get such a penalty if you were using Ride to actually ride a horse.