I found the following info, it might be of help for your calculations:
We know that 1.8 grams of CO2 == 61.02in3 so (12/1.8)(61.02)=406.7Vin3 at 1atm
We know that 1atm == 14.7 PSI so (406.7) * 14.7 / 150 = 39.86in3
So 12 grams of CO2 will fill 39.86in3 at 150 PSI at 77F.
Calculating the...