If I would have to guess, I'd say in real life the muffler pressure is a function of both throttle position and RPM (in helicopters, despite constant RPM muffler pressure rises and falls with throttle position, I know that much, and in airplanes, with constant throttle, muffler pressure also rises and falls with RPM, or the leaning out due unloading engine could not be counteracted with muffler pressure).

I would think a few testruns with measuring equipment should result in fairly generic ratios that allow for some sort of simple programming, analog to how the gas-law in the original controller compensates for temperature and barometric?