Well, the DC is set so it should require a roll of 10 to succeed, assuming you have +3 due to INT and 3 ranks at 1st level, and add 1 rank each level (55% chance). Now, this does not count having your familiar on hand (+10% chance), casting spell you have spell focus with (+10%), or a spell within your specialization (+10). If these factors are added in, the wizard has an 85% chance of success. Every level you are above the minimum level to cast a spell of that level adds +5%.. Every extra round spent casting the spell lowers the DC by 2 (+10%) – and also reduces the spell level for purpose of extra casting and damage for failing spell craft roll. The mention of +4 DC to ignore damage (and remove chance of bonus casting) increases the DC of the wizards spell craft check, not the spell's DC.
Example: take a 5th level wizard, non-specialist, no spell focus. He also forgot his familiar. He has a 16 INT and 7 ranks in spell craft (so a roll +10)
To cast 0th: 5+ roll.
To cast 1st: 6+ roll. If roll is 11+, gain extra casting of 0th level spell (use pearl of power mechanics)
To cast 2nd: 8+ roll. If roll is 13+, gain extra casting of 1st level spell
To cast 3rd: 10+ roll. If roll is 15+, gain extra casting of 2nd level spell
Failure to make roll – you still cast the spell, just you take subdual damage = 2*spell LV.
The reasoning behind it is: spell failure chance makes it more then point and click for the wizard, adds a little tension, and encourages the casting of spells withing your specialty. It also gives another function for the seldom remembered familiar (familiar can not be hidden away, must be out in the open and in contact with caster). The balance or payoff, the caster can gain extra castings of spells per day, assuming good rolls! For flavor, the caster who just gained the ability to cast spells of LV X may spend an extra round or two to get the spell off, while several levels later, his skill level has far exceeded the risk of a meager Xth level spell, and so whips out fireballs/magic missiles/what ever without concern. But when he casts spells of LV Y, he again takes it careful.
I'm still not sure about the #'s - the campaign I'm using will prevent an over abundence of 18's and 20's in the stats, but a game w/out a way to keep PC stats within reason may see this system as a disaster. I'm still in playtesting. Also note: I've eliminated the sorcerer in my game, otherwise this would step on their toe's.
