The "bonus" colum lists the current rating
Ok.
That "10 + Magic + Attribute Modifier" should probably just read "Magic + Attribute Modifier."
Ok, but then won't that imbalance things unduly at the lower-level end of things, with just about every spell being avoided by even lower than average D20 rolls? For example, our struggling 10th level Blm (yeah, I know, not
that low level, but bear with me) now has a MAGIC of 6, granting an MC of 6 (remember, he's none too bright for a Blm

), which is resisted by his evil twin Genome Blm-10 who has a RES of 3, meaning he only has to get a 3 on the D20 roll to resist our Hume's spells (DC 6 vs. D20+3).
I think that the "10+" formula works best; it just seems that the RES scores for characters don't keep up with their MAG scores, which, depending on the job, is more or less ok, so long as there is even a point of picking up the die. Which classes have the best RES to magic will have to rely upon your game theory and interpretation of the source material, but if there is the chance to resist, mechanically, then there needs to be the possibility for success,
especially in those Jobs that you deem to be capable of successfully being able to resist magic. For example, it may be decided that Jobs with low magic use are just incapable of resisting any but the most elementary spells, so some might possess this gradual degeneration of resistance, and have to suffice with other means of survival, like lots of HP or HP recovery methods. This certainly seems to be present in some of the FF incarnations, wherein you just have to have a ready supply of X-Potions available for use. Powerful magic just might not be capable of real resistance; but I think, for the D20 system, such might be better relfected in the results of successful resistance (ie Save for 1/2 rather than the Save to avoid or "negates" of less "powerful" spells), rather than the eventual impossibility of even a partial success, as this seems to be how the (d20) system is "balanced". But, thats just my half-mill on it.
Another aspect for D20 that is different than FFZ is that the spell save is based on the level of the spell, rather than "Caster Level", which given that spells capp at "9th" level, that means the save DCs are much lower than FFZ's potential, which has the corresponding difference in effect of the (essentially) same mechanical ability to resist, as reflected by class/job based save/RES modifiers. A +12 Save bonus means a lot more in D20 than a +12 RES stat because of the lower basis of spell/effect DCs for D20 spells than FFZ. [OTOH, I often find the spell dcs in D&D to be
too low for the challenge ratings they should represent]
I guess the solution will have to rely upon your vision as to how able you want characters to be able to resist magic. I like how you've got things set up (caster-level based DCs, single saves, ect), and think the present system you've created will work, just need some minor tweaking to the "hard numbers" for some fine tuning (which is what testing and drafts are for, Neh?)
BTW, so far, FF12 is
Kewl.....