[JesseK]

The relevant equation here is the ideal gas law, or P1*V1 = P2*V2, supposing that temperature is the same and no gas leaks.

60 PSI * cylinder volume = 120 PSI * stored volume

If you presume storage pressure to be around 100 PSI near the end of the match, you will want to calculate and test using 100 PSI for storage rather than 120 PSI. This is very likely if you use more than a couple of pneumatic-driven mechanisms.

Separate from storage, there is hysteresis when determining needed cylinder force. It is something I know exists but I don't know how to calculate. Effectively, it takes more force to get the cylinders moving and to continue moving than it does to keep the cylinders held in place. In my experience, it takes about 20% extra force to get the cylinder moving (e.g. 180lbs of force for a 150 lb robot) and 5% extra to keep it moving (e.g. 158 lbs of force for a 150-lb robot). You can play with hysteresis by adjusting your working pressure after the cylinder stops/starts moving.