For those who may be perplexed and not to teach anyone to suck eggs. Chasing altitude for altitude sake in these early jets is a waste of time. The wing design and engines were quite different from today's aircraft with supercritical wings and bypass fans.

Turbine and all engines for that matter, performance is rated on temps and pressures in the standard atmosphere. That is at sea level +15 degrees and 1013.25 hpa or millibars. Temperature decreases at a standard rate of 2 degrees per thousand feet and pressure by 1 mb per 30 ft. These decreases continue until you hit the tropopause where it starts to warm again.

The performance charts are designed to tell you how much you can load at the point of departure and what you can expect as a climb rate and what should be your optimum cruising level this is for planning purposes only. If you get to cruise and find is colder then you can probably squeeze some more altitude out of the bird but the tradeoof is fuel burn or lbs/nm. On a long distance flight the expected winds become critical as does the need to carry reserves.

Therefore you need to know what the current temp is at the departute point to determine whether the upper atmosphere is going to be warmer or colder than ISA? Generally tables for turbines will have 5 degree blocks but if you have a look at the 707 in the 707 perf.zip recommended by MarkOz then you will see 10 degree blocks. Not hard if is about 10 degrees warmer then it will be like that all the way up and a rough extrapolation will do.

PS - This is why not having the OAT in the Captain Sim 727 was such a pain, never knew what the temp was except by using Shift+Z.