NavList:

That's it !

From A/C GW * g = 1/2 P γ M**2 S, or in other words GW = k' * P (with P = Static Pressure, all the rest being constant, M=Mach Number, S=ref Surface), we do end up with dGW/GW = dP/P.

In the previous example dGW/GW = -8 tons / 280 tons = -0.0286 , which is something realistic with modern A/C and superhibypassratio Jets (about 7 to 1).

From a standard ISA table : 

starting at 35000', with P/P₀ = .2353, one hour later the new ratio will be .2353 * (1 - .0286) = .2286, which puts us close to 35750', yielding a Vertical Speed Indicator of +750'/h or very close to +13'/min ...

So a "climbing cruise" is beautiful and nice on paper, but probably still out of practical reach because of the so minute rates of climb at stake too small then to be manageable for a standard autopilot.

1000' or even 500' stepclimbs are easily achievable and almost as efficient as any continuous climbing cruise ... as long as you keep a very close tally of the AC GW, which we can instantly get on the A345 and most certainly on the B787 and A350 too.

Good to keep you F/O's busy with 500' stepclimbs, but I personally was way too lazy for that, and only climbed once per "1 hour + 15 or 20 minutes", so as to stepclimb by exactly +1,000 ft, while often "cheating" through setting the VSI at just +100'/min for 10 minutes in order to prevent the horses - already close to their thrust limits then - from going full power.

Kermit