So going off memory here as I don't have my book in front of me at the moment. The penetration for a might roll is
Might - (5x points of power used)
So a creature with a Magic Might of 20 uses a 2 point power against a magus and the penetration total is 20 - (5 *2) 10= so 10 points of penetration left. Now does that final 10 also get a stress die added or is it just straight 10 points?
RAW without die but + aura and penetration ability if it has it.
Thanks.
Also add the value of Penetration ability, if the creature has it.
Plus, note that some Creature Powers/Qualities reduce the Might cost of Powers effectively giving a better Penetration.
IIRC a creature's Penetration is based in Might Score, so it is based on an unchanged number. But Resistance is Might Pool, so using Powers - or being affected by Might Stripper spells - chiåps away at the creature's defenses. Althjough I might be wrong, so please correct me and post book/page references if this is the case.
The very last two lines of the section "Creature Might" ArM5, p.191, write in bold the simple formula for Magic Resistance:
"CREATURE MAGIC RESISTANCE: Might Score".
The Errata changes this into Might Score + Aura Modifier.
So Magic Resistance is based on Might Score, not Might Pool.
Though I would probably like it better were it based on Might Pool.
Just be sure to use the aura in both places. Players tend to remember to use the aura in their spellcasting, but I've bene known to leave it off of the resistance calculation. Also, it becomes much more important areas where there is an adverse aura... Magi casting spells in a Faerie aura add half the aura, while the faeries there get the full benefit of the aura...