You leave some unanswered questions. In order to calculate cylinder pressures you need to take the atmospheric pressure at sea level, about 15 psi, and multiply that by the compression ratio. For what you are doing I would add back in the 15 psi, remember you didn't start with zero, which would give a total pressure of 90 psi, divided by 15 would give a compression ratio of 6 to 1. Your engine should run with that. Now for the questions, what was your engine temperature when you did that test? Were all 4 cylinders 75 psi? You did have your engine running recently didn't you, certainly at some time prior to doing a test. What were the results of the cylinder leak down test? Was this compression test wet or dry?
Take care!!
Andy