Pressure drop thru pipes and fittings of compressible fluid

[presso]

I'm using equations 3.20 in Crane to calculate the pressure drop of a compressible fluid at a give rate through pipes and fittings using values of the net expansion factor given in page A-22. However, most of my calculations involves propylene which has a somewhat lower Cp/Cv (= 1.15 roughly) than the lowest value of page A-22, that is Cp/Cv = 1.3.
Is there a way to obtain similar data for other Cp/Cv values? I've done a small amount of research on this subject and i still didn't find any clue, only data for Cp/Cv = 1.3 or 1.4! Else, do you think the difference of using a value of 1.3 instead of 1.15 or closer could be important?
Thank you!