You maybe correct IO540, but you're forgetting that most of us pilots are creatures of habit! We fly generally fly with the same RPM or RPM/MP combination!
I reckon that's why it works, and am very conceous that if I vary that or fly well beyond my usual altitude bands, then all bets are off.
That was also why I was determined to get my own fuel usage figures. They are relevant for how I fly the aircraft, and not some figure for how someone else flys it (or has remembered from someone else who told them, who heard from someone else who ought to know, etc etc).
dp