For air at ambient temperatures, you can assume it to be an ideal gas (don't go over a couple hundred psi if the results are critical with this assumption or go to very low temperatures).

P1/P2 = V2/V1 where P1 and P2 are the ABSOLUTE pressures and V1 and V2 are the volumes.  To convert from psig to absolute pressure, add the atmospheric pressure (14.7 psiA at sea level) to the gauge pressure.  For example, 100 psig would become 114.7 psiA at sea level.

Doubling the absolute pressure will half the volume and so on.

Here's a table out of Spirax Sarco's "Compressed Air - Practical Study". For example, if you had a compressor delivering 200 cubic feet per minute of free air, then at the compressor discharge pressure of (for example) 100 PSIG, this would be 200 / 7.8 (from the table below) = 25.64 cubic feet. The ratios are determined using absolute pressures. At 100 PSIG, or 114.69 / 14.69 = 7.8.

PSIG  Ration of Compression

10    1.68
20    2.36
30    3.04
40    3.72
50    4.40
60    5.08
70    5.76
80    6.44
90    7.12
100   7.8
110   8.48
120   9.16
130   9.84
140   10.52
150   11.2
200   14.6

Thank you very much, your help is greatly appreciated.