I did level 0 for all rr's but eventually settled on resistance level is equal to relevant stat mod. So a +8 In mod equals a level 8 resistance versus channeling. Attack levels are equal to ranks in skill/spell list. If Spirit Mastery is known to 9th level that all casting and attacks are calculated as a level 9 attack.
This works with all skills. An attempt to disarm for is figured with attack level equaling skill ranks in disarm foe and resisted versus foes ranks in weapon. Stat mod x3 for both attack penalty and resistance bonus, which can vary but are usually St (a Brownie using disarm with a skill bomus of 80 and 13 ranks in the skill would use Ag to attempt disarm but resist with St...ymmv).
Resistance levels are fairly static, but I do have rare and hard to find Resistance skills, one for each realm of magic and another for poison. More common are charms providing a level bonus or modifier to resist. Certain helmets for example have a unique enchanted metal that provides a level bonus versus mental attacks. Certain creatures to have innate resistances. Demons in my game are highly resistant to essence, resist essence at level with a bonus equal to typex10.