The OP is flying a light aircraft AFAICR. He is just trying to emulate the (somewhat imprecise) performance figures derived by the manufacturer test flying program, done eons ago on a brand new aircraft with a TP at the controls. Even with the manufacturers and regulators adjustments to cater for more realistic performance of a production aircraft with a moderately proficient pilot the performance figures are pretty vague at best.
If he follows my suggestion he will be able to derive a formula that will be well within the accuracy that can be derived from that manufacturers graph, just given the thickness of those ink-drawn lines…
The great advantage of my suggestions is that it self-adjusts as you get more data. So if the results of your derived formula disagree with what you calculate from the FOM data - just input the new data point into your data base and the polynomial constants will adjust. But reallygiven the simplicity of that performance graph (largely linear with temp and wind, maybe second order with mass) a very simple formula derived from 20-30 data points will be very accurate (just remember my warning about extrapolation outside of limits!).