I don't know what else could be used, there isn't really another formula that's going to give you any better or any significant different (advantageous) out come. You can adjust the efficiency rate in the long version to adjust to different applications and conditions. Combine efficiency rates take into account any down or idle time, opening a field, turning on the headlands, unloading, stopping to pick up the land lord, neighbor or kids to ride around, when Nature calls, the auger cart is full and all trucks are gone, etc. That's why I think 75% is more realistic rate of measure for harvesting. 82-85% is fine for tillage, etc. because you aren't relying on the support crew like harvesting. In harvesting, everyone has to run at 82.5%, not just the combine. Might take some of the fun out of it if so.