Greetings . . .
90 PSI is a decent number for one of these old engines.
Measuring compression ratio with a PSI gauge depends on the starting the point (barometric pressure) - which is dependent on ambient temprature, engine temp, humidity and elevation with respect to sea level. At sea level with a steady barometer, one atmosphere is roughly 14.7 psi.
So if your car is sitting with a cold engine at sea level then your 90 PSI reading is about a 6:1 compression ratio - or six atmospheres. Spec would be just under 100 PSI. If, however, your car is sitting in Denver CO the starting pressure is lower so your CR would be higher with 90 PSI reading.
Here's an online pressure calculator that you can experiment with:
PSI to Atmospheres Converter regards,
Keith