I can give you my opinion from a pvm perspective. I rely heavily on casting rising colossus, which even at 120 is 99% success rate. The summons doesn't last very long either, so when I need to recast it, I don't want to fail at it. I would think if you're not going to be casting that so much, you probably could get away with less than 120. Another note is that rising colossus is also impacted by imbuing and eval. I have all three at 120 and rising colossus is pretty powerful.