How can I find out the acitivty needed for legendary for a certain account?
AFAIK, you can't. Just somewhere between 775 and 1030 activity.
AFAIK you can, theymos said that atleast. Maybe only staff can look it up though.
He actually explained that going to legendary is a random number between those values, he hashes a salt and someother stuff to calculate the value and everyone goes legendary at some unknown point in that range. Staff won't look it up (don't think they even can) cos it's supposed to be secret.