All signs point to that piston speed range as a good proxy for when you don't have a BSFC chart.
Years ago I inquired here about optimal piston speed and why wouldn't slower be better (because the calcs showed my F150 so different from the cars and bikes)? There is a sweet spot between too slow (excessive heat loss through head, piston, cylinder walls) and too fast (higher internal friction).
https://ecomodder.com/forum/showthre...peed-1477.html