% of power lost in drivetrain

Can we really use a set % for all engines? If we use the 20% on a 450HP motor with a 727, that gives the approximate rear wheel HP of 360. So, if we had a 600HP motor, this equates to 480HP to the rear wheels. It seams that there should be more of a set number required to run the drive system. If we use the 20% rule, why does the 600hp scenario require 120hp vs 90hp from the 450hp scenario. Just wondering what the thoughts are on this, thanks.