Range Factor
Over the years, whiling away the hours I used to try and back-figure what wetted area, roughness, drags, etc were. Goodly enough to blag my way into working for the makers rather than the flyers when the time came. Here is a strange thing... or maybe not so strange. On a wide spread of makes and marques including the B737-400, B767-300, B747-200, B747-400, A330-200 and A319-100 a common factor emerged. It is a rare bird that performs well aerodynamically in cruise (and mated to the engine) above lift coeffts around 0.56
I really ought to have got out more often but I ended up with wodges of comparative data that I rendered into range factor = (V/c)*(L/D)
Of course to know "c" you have to know "L/D" but it is not the individual values that matters so much as the product. All joking aside but I learned when to step and when not to step and usually got my fuel burn about right
Okay, c of g and trim and all that I know too. Just observing that as soon as the range factor next available level looks to be better and there are no penalties such as wind (pardon vicar) and you have at least an hour of cruise left, give it a shot. But if the lift coefft reqd would be high-ish and it is above FL390, I might be inclined to give it a rest.
What I mean by high-ish is hard to define other than I have given it a lot of thought in a lot of hotels. BTW I work for that other firm now. There is a Dog!
Best Rgds
The "E"