In case it's of any interest, we used to "thumb" through a rather well-presented cruise performance booklet in my VC10 days. Oddly enough, I don't remember it including a graph for "Optimum Altitude", as such - unlike all the jets I flew subsequently. (Yes, the last one also presented it in the FMS.) Here's a link to our VC10 page for M0.82 (the other pages covered up to M0.86).
http://www.pprune.org/aviation-histo...ml#post8275311
I guess one definition of Opt Alt, for a given Mach, is the altitude at which the fuel burn is least for each AIR mile. The graphs show that as KG/N.A.M. (nautical air mile). For each of the various weights, the curves of altitude versus KG/N.A.M. bottom-out at a buffet margin of around 1.4g. As you can see, the "I.O.A.T." (TAT) apparently makes no difference, because the higher fuel-flow in hotter air is offset by the higher TAS at the same Mach.
Needless to say, the kgs per nautical GROUND mile at the proposed new F/L are another matter. You never really know what the wind is going to be up there - unless some preceding crew can tell you. And that's particularly true when you are near the trop (er, tropopause).
I should point out, however, that the VC10's Conway engines had a very low bypass-ratio (hence the excruciating fuel-flows)
, and were powerful enough to get you well up into coffin-corner if you were tired of living - hence the pecked lines for the 1.35g, 1.25g and 1.15g buffet-margins. We tended to wait until we could climb on the 1.35 buffet, unless we were going to lose an important opportunity from ATC...
PS
At risk of stating the obvious, perhaps I should add that the above considerations don't relate to the optimum altitude for the short sector or diversion case. Even if there are sufficient track miles to climb to optimum altitude and descend to the destination, the benefits of doing so are less pronounced for a high-bypass turbofan than with a low-bypass engine or (particularly) a turbo-jet.