Can't find anything really concrete in the books. Is there a hard and fast rule for the drop in ambient mana as you get further from earth?

SM, p119:
"As you get farther from the Earth’s astral form and the presence of living material, however, astral space becomes one vast void. At the upper mesosphere of Earth’s atmosphere and beyond into deep space, the astral plane is a singular void..."

Wikipedia says this about the Mesosphere:
"The exact upper and lower boundaries of the mesosphere vary with latitude and with season, but the lower boundary of the mesosphere is usually located at heights of about 50 km above the Earth's surface and the mesopause (upper boundary of the mesosphere) is usually at heights near 100 km, except at middle and high latitudes in summer where it descends to heights of about 85 km."

As for step-wise drops in the mana? I'd do something linear, from 0 to -12, depending on altitude. Make it easy and say -1 per 10km, with total void of -12 at 120km?

That might actually work pretty well. That pegs Mt Everest safely within the the thick manasphere, and even most subsonic slights should be fine. Everything higher is generally non-passenger air traffic, suborbitals, or is outbound for space.

Magic in the Shadows p. 86
"The Earth's aura(known as the Giasphere to magical theorists) extends to the edge of the atmosphere(about 80 kilometers up.) At 71 kilometers the background count is 1 and increases by 1 for every additional kilometer of altitude up, up to a maximum of 10 at 81 kilometers up. At this height the atmosphere is a total mana warp"

Wake of the Comet puts the background count of the Apollo space station at 9, probably because of the plants and folks up there.

Wow, that's... a pretty sharp curve there.
I've seen some example of stuff out on lunar bases with a BC of 9, so yeah, it's probably the small amount of life there giving off low levels of mana.