So I'm a big fan of the Target 20 method for determining hits ( in AS&SH the equation is:  FA/HD + Modifiers + AC = 20 or higher = Hit) so I took a look at the spell resistance ratings and tried to see if I could do a similar thing with the casters CA and make the math easier for me in running combat.

I started with the common 7 in 20 chance of spell resistance vs a CA 12 caster. Since for every CA level less than 12  the chance-in-twenty increases by a one a caster with CA 1 would face an 18 in 20 chance of resistance.  To reverse this number the CA1 has a 2 in 20 chance of affecting the creature (and the CA 12 a 13 in 20 chance).  Therefore for a 7 in 20 spell resistance you can use the equation of:   1 + CA level = chance of spell affecting in 20.  I think this formulation makes for easier math and you can have the player roll if you like so they feel like they've defeated the spell resistance.   Thoughts?   Is my math correct?