Going by the Cockpit ToolTip I set my throttles at 70% and the flight is usually around Mach 0.8 - 0.82 as well. I'm not sure what the N2 readings are though. The contrail seems to have the black smoke mixed in with it, but to a much lesser degree than say when the contrail first start and the throttles are further forward for climbing.

In cruise, if I have a powerful tailwind, I pull the throttles back further and with a headwind, I push the throttles forward further, obviously and the black smoke will sometimes disappear altogether after throttling back quite a bit. I'll have to do a flight and take some screenshots to show what I mean.