If you totalize 400 magic evasion and the mob has 400 magic accuracy, you resist 50% of it's spells. If you downgrade to 390 magic evasion, you resist 40% of the time, with 380 meva you resist 30% of the time ... With 345 meva you resist 5% of the time, this is the cap. If you increase your magic evasion past 400 it's half has good : with 410 you resist 55% of the time ... with 490 meva you resist 95% of the time.
The hard part is reaching the mobs' macc minus 45. For info at 75 even match mobs had ~ 275 magic acc (let's say BLM mobs) while you had ~130 base meva. At level 90, even match mobs should have ~ 360 magic accuracy and you should have ~ 215 magic evasion (~ +4 meva per level). This means that you still need +100 magic evasion to start seeing results. Besically barspells from RDM are about enough to make the next 50 elemental resists match -50% magic damage taken and the next 100 another -50%. At higher levels mobs will have ~5 magic accuracy gain per level probably (similar to players) so against typical +7 level mobs you need ~35 more magic evasion :
weak mobs (<= level 90) : barspell + x elemental resist = -x% magic damage taken if x <= 50 and -50% -(x-50)/2 % if x >=50.
strong mobs : require ~ 270 to cap out resist, 180 to resist half of them.
What's cool with magic eva is that you can almost nullify the damage unlike typcial mdt.