Given that by definition of scale height an atmosphere thins by a factor of **1/e^x** where **x** is elevation in terms of scale height multiples (See the table here: [Definition of Scale Height][1]), can we assume that the atmosphere is effectively non-existent at the elevation of 6H?

Density at elevation 6H
1/e^6 = ~0.00248 would mean about 0.2% of density at surface level


*This is a followup to another question:
[How can you determine the initial volume of a planet's atmosphere?][2]


  [1]: http://astro.unl.edu/naap/scaleheight/sh_bg1.html
  [2]: http://astronomy.stackexchange.com/questions/10279/how-can-you-determine-the-initial-volume-of-a-planets-atmosphere