Goal time / percentage (in decimals)
If you can run a 50 in 6 seconds and want to run it at 75% intensity:
6/.75 = 8 seconds
We build up with some reps at .50/.60/.70 as warmup sets to get a feel for what each intensity feels like. Eventually you'll be able to guess within a second how close you were to your goal time.
If we are running a different distance, we just figure out the time needed for the longer distance and divide accordingly.