quick answer
If you mean common everyday expectation and not MMO then I’d guess M0.80 as you say. On my last acquaintance, when flat out economy was the driver then M0.77 and a lift coefficient of 0.55 would sit nicely at FL330. Sacrificing a little but with an eye on good long range performance over ca. 3900nm then M0.785 perhaps with a lift coefft of 0.53 would do nicely. High speed? Then M0.795 and a lift coefft of 0.52 seems in order. Fuel load would be based around ideas like these.
In general the B767 does not like to go above M0.80 in cruise because the L/D starts to reduce rather sharply because compressibility drag for one thing starts to increase very rapidly. Whereas induced drag follows a square law, compressibility has terms that can be modelled in cube and quad laws. I say "modelled" because the most commonly used interpolation formulae go to third and fourth powers. What the aerodynamics is doing is another thing but the mathematical fit of these interpolation equations is good.
I take it as read that if you ask for more than M0.80 you mean short duration for pressing ATC reasons otherwise the route fuel calculations are liable to go out the window. But you know that.