...Anyway back to the OP and specifically :
I've been reading about why Jets climb as high as possible,
FWIW ..we certainly
don't routinely climb as "high as possible" (i. e. Which in our case would be FMC max)..,,as I recall it years back from CFS days design RPM (on a small turbo jet ) came into the best flight level equation....and suspiciously enough these days (and ignoring winds) our best flight level on the 777 (and I think similar was true on the 744) seems to where the N1 was around 90%.....and usually a thousand feet or more below "as high as possible" ..... in my old age am I misunderstanding best range/design RPM?