Most basic question about resistance possible

Just learning the game here, and it's not a stupid question if you don't know the answer. But, in forth edition, what is the answer to the following question, and more importantly, where do you find it in the book. Ther index says resistance is described on pages 72-73,247, but having read those pages repeatedly, I still have no clue.

The magus gets hit by a spell that magic resistance works on. The player rolls a stress die (let's say it comes up a 4), adds his resistance from Parma (call it a 15 from Parma 3), adds in the form score (a nice 13, why not). That gives a total 20. What do you do with that number?

The description of casting a spell says to remember what the casting roll was because it's important for penetration, but not what exactly to do with it. Under the penetration description, it says you compare them, but nothing further. Do you just see which is higher? Do you subtract resistance from the casting roll? Do you add the two values and give the GM that many silver pennies and a poke in the eye?

I can think of several different things that make sense, but without crunching the numbers I can't make an educated guess as to which is the actual rule, and I'm going crazy looking for it.

You're playing fourth edition?

If so the casting total + penetration skill of the caster must exceed the resistance.

If you're playing fifth edition the casting total + penetration skill - spell level must exceed the resistance (which in fifth edition doesn't get a die roll).

The fifth edition rules came out of a commonly used fourth edition house rule so I included them for your consideration.

I would add that Magic Resistance in ArM (Fourth Edition of Fifth) is an all-or-nothing affair: either the resistance keeps the spell out entirely, or lets it completely through. I am not sure that is clearly stated in the ArM4 rules.