Ideal gas law is express as

PV=nRT

Where P is pressure ("Hg), V is volume (ft^3); n (moles, lbs/lbmole); M (molecular weight, lbs/lbmol); R (gas law constant = 21.85 ("Hg*ft^3/lbmol/deg R); T (deg R). However, there is much more involved!!!

What you might be looking for is an altitude density calculator. Search the Internet and you can find tons of them. My favorite is http://www.wahiduddin.net/calc/calc_hp_dp.htm

We rate JetCat engines for a standard day. The ICAO International Standard Atmosphere standard conditions for zero density altitude are 0 meters (0 feet) altitude, 15 deg C (59 deg F) air temp, 1013.25 mb (29.921 in Hg) pressure and 0 % relative humidity ( absolute zero dew point). The standard sea level air density is 1.225 kg/m3 (0.002378 slugs/ft3).

It is very difficult however to directly calculate the thrust change at different density altitude conditions. For example, the P160 is more sensitive to density altitude compared to P80s or P120s. This means the P160 is closer to the edge. This is true with many other brands. The best way I have found in testing new engine models is doing it empirically. I test them here in LA for the closest readings to a Standard Day and then send the engine to MTSS, our dealer in Denver, to test them at 5,400 feet. The differences are quite significant!

I think you will have a hard time calculating the exact thrust difference for different altitude densities.

Best of luck!