Here’s how 1073 did this in 2010 for our variable pressure ball kicker.
Put a valve between the regulator and the pressure tank.
Put a pressure sensor on the pressure tank.
Pulse the valve, in roughly 1 ms increments until you get the pressure you want.
I also recommend putting a 2nd valve on the pressure tank side, which vents the tank. You can pulse this valve to drop the pressure.
This seemed to be repeatable within a couple PSI.
You will need to have two manual vents, one on each side of the first valve, to completely depressurize the system. Or, you can wire the 2nd valve to automatically vent when you lose power.
This system can also be used to pre-charge a cylinder that is restrained half way. In this case, the automated venting is vital for safety, since you do not want that stored energy in the robot when it is disabled.
Now, I am fully aware that these valves were probably not meant to be used this way, but our system was very reliable. We used the large plastic Festo valves.
Hope this helps.
-Jeff