Tying casting to a skill check [PF]

[Svm420]

If you were tying casting to a skill check how would you set the DC?

How would you set DC so that success is never more than 75% to 80%, even at level 20, assuming high but not perfect optimization for the skill in question and its a class skill. Thanks in advance!

[OldTrees1]

Start by seeing how the skill modifier would scale.

1d20+(L+3)[Class Skill]+X[Ability Modifier]+3[Skill Focus]+2[circumstance from Masterwork Item](+0 to +30)[competence from custom item]

The ability would likely be (16to18)[base]+2[racial](+0to+3)[Age](+0to+6)[enhancement](+0to+5)[Levels](+0to+5)[Tomes]

If we assume:
(Note: it is likely that some of these assumptions are wrong. This is merely an example)
Custom item gives a bonus of +3L/2.
16 base stat
+2 racial
+1 (Middle Aged)
+6 enhancement and +2 tome estimated as +2 every 5 levels
So ability modifier would be +4.5+L/8+L/5
So skill modifier would be would be +(L+3)[Class Skill](+4.5+L/8+L/5)[Ability Modifier]+3[Skill Focus]+2[circumstance from Masterwork Item](+3L/2)[competence from custom item](+L)
So skill modifier = (1/1+1/8+1/5+3/2)L+12.5 = (113/40)L+12.5
Or about 2.8L+12.5

Also read up on Truenamers. WotC screwed up their attempt and it would be good to know why when trying the same idea.

[Ninjaxenomorph]

Yeah, its pretty easy to max out a skill. It would probably be a main casting stat (+4/5 at lower levels), max ranks, class bonus, Skill Focus, then item.

[Svm420]

Thanks for the input I think a DC of 15+1.5lvl rounded up works for me with low to mid op