fuindordm said:A softer option:
15+spell level gives a DC range 15-24
Full concentration ranks + combat casting +1 Con - armor check penalty of 4 (breastplate) = a bonus of 4+level to the roll. This is still pretty tricky at low levels, and without yet more investment such as taking the skill focus feat, remains so even to the mid levels. At 10th level you get +14, compared to a DC of 20 for your highest level spell; at 15th level you're at +19, compared to a DC of 23. So you get better and better, but you still have a non-negligible failure rate.
What this amounts to is saying to the armored spellcaster, if you want to wear something better than Mage Armor all the time, you need a high Str, max ranks in Con, and Skill Focus (Concentration) to beat your failure rate down to the 5-10% range. Mithril armor will help a lot, of course, but at a DC of 15+spell level you're making it highly likely that they'll take the feat to wear armor.
In which case, you might as well just use an Armored Casting feat.
A DC of 10+spell level, on the other hand, renders the check trivial once you hit mid-levels; you'll see mithril-breastplated spellcasters much more commonly. If you think a mithril breastplate with no spell failure is more valuable to the spellcaster than one level of lost spellcasting, then 10+spell level is
probably not enough.
Ben
What I'm suggesting is abandoning the Concentration (or any other skill-based) mechanic. Otherwise, you are dealing with a check that goes up every level against a DC that goes up every other level (and only for the highest level spell). So, either you make the DC extremely high so that high level wizards are still discouraged from casting in armor, and at the same time discourage anyone from playing a low-level cleric or druid (because of significant chance of spell failure). Or, you make the DC low enough for armored divine casters to cast spells easily, but make it extremely easy for high level wizards to cast in armor. (Why, by the way, would a wizard necessarily take a level of fighter for the armor? As long as they are willing to take on the check penalty to attacks, they don't need the proficiency.)
I appreciate the discussion and the points of view regarding internal consistency, etc., but ASF, IMHO, does its job: it preserves the image, sacred cow or not, of the unarmored Gandalf or Merlin as the iconic wizard. If the "correction" involves a more complex mechanic, it does not seem worthwhile.
--Axe