For those who don't know AP10 he is a pro 8 tabling the $200 tables and plays about 20k hands/week - he therefore has a huge and up to date SSNL database for full ring - he also helps out folks like me and PokerAce when we are analysing player stats and need data (he was instumental in the new PT autorates for example and I think helped Poker Ace with the new aggression stat) and he also spends a lot of time looking at it himself - so if he says 50% one thing is sure - that isn't just a 'guess'.
There is a thread on the BTP PokerTracker forums started by PokerAce and contributed to by AP10 in which he says:
'I did extensive research while creating this formula and am continuing to do research trying to determine a weighted system. With that said, using the current weights of one for each, the optimum aggression is as follows:
Flop: 45%
Turn: 50%
River: 50%
These are the maximum's when graphing BB/100 vs Aggression Frequency <street>. '
the thread is linked here
http://www.bet-the-pot.com/forum/viewtopic.php?t=6517