Intensity is % of 1 rep max but bodybuilders seem to think it's training "hard" or to failure. Training to failure is not optimal at all.
your 1 rep max can vary from day to day
I day you may do 400 on the bench but then may not get 380 for the next two weeks
your muscles dont know what a 1 rep max is
I have no idea what my one rep max is because Im not risking injuring myself trying to find out