How slow of a spellcasting progression should a versatile at will caster get? [3.5]

[OldTrees]

I am also planing on having there to be an ability check to cast spells

So if I go with
a 10% failure to cast 4th
a 50% failure to cast 5th
a 90% failure to cast 6th
at 20th level that would sound balance?

(Fullcaster spell level -> unchanged) Ex. Magic Missile Wiz 1st -> 1st
(Halfcaster Pal/Rgr/Ass spell level -> +1) Ex. Holy Sword Pal 4th -> 5th

Thank you.


By the way what would be a good estimate of ability modifiers as a function of level for this DAD caster class?