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85% is the minimum distance, but lens quality improves the further out you go because the lensing region moves up into the stratosphere so you get less distortion from clouds.

The moon's gravity is in play so the orbit might not be so simple, but there are all sorts of "halo orbits", orbits around the L4 and L5 points, etc.

It seems to me though that the Earth's atmosphere is always going to be "lit up" one way or another by some phenomenon like the Aurora Borealis, scattering and various sorts of skyglow, so there will be some background, but maybe it is not that bad.

Apparently at the distance of the Hill Sphere of the Earth (about 4 times the Moon's orbital distance) the effect would still work.