GRLWEAP - How to calculate "Pressure"

[JHeisenberg]

I know Energy = Stroke Height X Ram Weight.

I am curious how pressure is calculated in the Hammer Parameters. For example, in the GRLWEAP hammer table a pressure (psi) is provided for each reduced fuel setting. How is this calculated?

If I am provided a fuel setting of energy XX (k-ft) from a contractor, how do I compare this to GRWEAP fuel setting of pressure XX (psi).

Thanks!