Spell failure chance is terrible. Almost no arcane casters go for it. Also, any mechanic that requires an extra roll in 3e -in this case, a percentile spell failure chance- is a bad one. Instead convert the percentile chance to a flat penalty to spell DC at a rate of -1 for each 5%. You may need to ad hoc a rule that arcane casters can't wear heavier than light armor, but I kind of doubt it, as spell DC is pretty solidly balanced by CR.