What? Please explain 
75% off max speed
100% = PB - lets say 10sec
50% off 10sec = 5sec. 5sec + 10sec = 15sec
75% off 10sec = 2.5sec. add your 10sec = 12.5sec
The diff between 75% and 100% is 25%.
What we are not doing is
10sec = 0%
50% = 5sec + 10sec = 15sec (this is the same)
75% = 7.5sec + 10sec = 7.5sec
100% = 10sec, + 10sec = 20sec.
which still isnt what your talking about. Perhaps iv been working too hard in the Sun today!