Skill Check

Success = 1 - 1 / (1 + exp(0.03 * (Attack - Defense))

From http://www.calloffate.com/forum/forum_posts.asp?TID=191&PID=29738 :

Answer: srand - Casting spells uses a slightly different skill check curve than that used for all other skill checks. It has the same midpoint, but is steeper through the middle. So once you are over the 50/50 point, your casting gets reliable much faster than with a normal skill check curve. But conversely, under the 50/50 point your casting is much less reliable than with the normal skill check curve. I just found this out tonight, myself.