Concentration checks for casting defensively

[motoko's ghost]

Would they work better as a base plus the spell level, rather than a flat DC?
Something like 15+spell level or 10+ 2*spell level.
Otherwise after lv 6 or so a wizard can make that check consistently and anyone in melee cant make AoO's against him unless they're specialised for that, which makes it harder for melee to compete with casters(please dont leave messages about how they cant compete anyway:smalltongue:)
It also seems to make a bit more sense that it would be harder to do so for higher level, more powerful spells.

So, would this work as a houserule?

[Yitzi]

Could work, but I have a better idea: The DC is 15 plus an additional amount chosen by the caster, and it still provokes AoOs (as if casting non-defensively) from anyone with a BAB higher than that additional amount.

That way, casting defensively is still possible if really needed, and also gives some protection from mooks, but when it comes to serious enemies he'd much rather have a friendly melee character around to keep the enemies away from him.

A word of warning, though: Any approach to making casting defensively not be an autosuccess at higher levels means that the melee ally needs to in fact be able to keep enemies away from the caster even at higher levels. My favored approach to do that is to allow (either with or without a feat requirement) a move action to ready a move action (and of course give the melee character some way to prevent monsters from just flying over his head); that way, the melee character can ready a move action to keep himself between the caster and the enemy.

[Zaydos]

If I wanted to make it not be an autosuccess I'd do something like DC 10 + 3 x spell level (meaning that with optimization you could autosucceed but that's DC 37 for a 9th level spell when max ranks only gives you a +23 + Con). 10 + 2 x spell level will still fall into autosuccess (if they have +4 Con after items). 15 + 2 x spell level won't unless they devote an item (Tunic of Steady Spellcasting) to it or have +5 Con (after items) and Combat Casting or +6 and Skill Focus (Concentration).

It would be a minor nerf to spellcasters and even an unoptimized one wouldn't be hurt too much.

[motoko's ghost]

15+ 2*spell level sounds best then as it means they still can do it, just not as easily unless they actually devote themselves to it. (higher levels=magic items slots are precious and they will have 2-3 things to choose from for each)

EDIT: It wasnt meant to be more than a minor nerf, it just kind irked me that avoiding the kobolds club long enough to cast a magic missile at 1stlv is the same difficulty as dodging the scimitars of a marilith dervish while attempting to cast a wish spell to replicate a spell you cant even cast normally at 20thlv.