Order-of-magnitude values for the density and pressure of the Earth's atmosphere are readily obtained for altitudes up to 160 km by means of a simple rule-of-thumb. The results, while not precise, are good enough for rough calculations when your favorite atmospheric table is not at hand.
The rule-of-thumb is atmospheric massdensity, number-density, and pressure all decrease by approximately a factor of 10 for every 10-mile increase in altitude up to an altitude of 100 miles. (This is one of the few cases in which working with English units is advantageous.) Interpolation in 5-mile increments is obtained by using a factor of three decrease in atmospheric parameters for every 5-mile increase in altitude.