I normally go with 315+ only to indicate they have done enough raids to not need hand-holding. Unfortunately that means after a few weeks that would need to bump up the over 320 level (not sure about 325+ though). However, if you have a team of all 317-320 you will run through the raid easily (as long as everyone knows what to do). Which is paradoxical since you are doing the hard mode to get higher level gear in order to do something that doesn't require higher level gear.