REALLY basic 4th edition magic resistance question

Hi all. I'm sorry to ask such a basic sounding question, but I've been looking through my 4th edition pdf for about an hour now, and in addition I've googled the question and checked all the errata and FAQs that I can find, all to no avail.

My question is this - How is the success or failure of magic resistance determined? All I can see is that you roll a stress die and add your magic resistance rating to it... I've looked and looked, but I can't seem to find what you actually compare that result to in order to determine success or failure.

Have I been looking in the wrong place, or missing a key sentence or half-sentence which answers this?

Again, sorry for the totally basic, trivial question. Any help you can furnish is appreciated.

The spell is resisted if your magic resistance + stress die is greater than the spell's casting total + the caster's penetration ability.

ArM5 changes that by subtracting the spell's level from the penetration total.

I guess that makes sense... is there any place in the book where it says this, though?

Thanks for the quick answer!

Hmmm... maybe I'm misunderstanding how you phrased it, but wouldn't that make higher level spells easier to resist?

This is a purely academic question as my Ars Magica gaming will be 4th edition based.

Under penetration on page 53. I know... it should have been more prominent...

EDIT: also on page 69 and 70.

Exactly. More of your power is spent actually casting the spell and less is available to penetrate any magic resistance. That makes both Parma and having a repertoire of low-level spells really matter.

You're welcome.