Can you assume atmosphere height for the purpose of surface pressure calculation?
It all depends on what you are trying to do. For some purposes 6H might correspond to an adequate vacuum  for others not. The density is not low enough to ignore its effects on a space craft (we are talking of $\approx 48$ km in the case of the Earth). Indeed $48$km is less than the altitude record for an unmanned balloon, though greater than the altitude records for air breathing aircraft.