I wrote this magic resistance ruleset a while back, and decided it was too complicated. A chance to avoid spells works better. Let's tie it into an existing mechanic.
You wear Armor to avoid attacks. This is called AC.
You wear Ward to avoid spells. This is called WR. Could also be short for “Witchcraft Resistance”.
Spells that already give a saving throw have no effect on ward, they avoid it entirely. The save is avoiding the physical or magical energy causing the effect. Spells that do not grant a saving throw must content with warding, as they are directly attempting to alter the target of the spell. Things like Sleep and Magic Missile therefore require a roll to bypass the Ward of a target.
The Ward of any creature is base 10, or base 1 if you want spells to function similarly with most creatures not being able to resist them. Wisdom modifier is added to this protection, similar to AC adds your Dexterity bonus. (Or Con bonus, if you're a hipster like me.) Casters roll a d20 + Int modifier + level of spell equal or higher to the Ward to hit.
Wards are increased by wearing a Talisman. In the same way you can only wear one suit of armor at a time, and better armors just increase your AC, Ward can only be gained from one talisman at the time. Homespun protective charms protect like leather, high end crystal talismans made by shamans protect like chain or plate.