90% of your PB: what time is that?
Running at 90% of your PB means running at 90% of your speed over that distance. So the time is divided by 0.9: with a 10.1 PB over 80 m, aim for 11.22.
Adding 10% to the time (10.1 + 10% = 11.11) gives a time that is too fast. The gap is small around 95% and grows as the percentage goes down.