Extrapolating gives surprisingly close numbers. We've checked the extrapolated numbers against the load factors on electronic engines and found a reasonable correlation. So simply we use fuel burn at actual speed divided by fuel burn at wot as an estimation. ICF and EPA even use a different and interesting calculation that I find doesn't come as close, especially at low loads. They use (Actual speed divided by maximum speed) cubed. There is actually a formula that combines factors and uses pressures and is more scientific and more along the lines of what ECM's use, but without an ECM or other equipment you don't have the needed information. I'll try to find that formula or have someone find it for me in the next couple of days. However, we have found that using fuel flow divided by maximum fuel flow is extremely close and good enough for the vast majority of purposes. I also reference Marmot's post #4 in 2008 in this linked thread. https://www.yachtforums.com/threads/calculating-engine-load-factor.8661/ We have performance charts we've built over time on our boats of fuel burn at different RPM and speeds and find that calculating load from those charts as described is very close. As to feedback on what the governor is doing, he's busy right now with coronavirus issues. Oh you mean the engine governor. Sorry. What the governor is doing is normally going to be reflected in fuel burn. It comes down to just using the best information one has and accepting it may be off a slight amount but not a significant amount.