An aside -
The majority of folks aver to the use of 30 ft/mb and 120 ft/C deg deviation when playing with pressure and density heights.
That's fine, providing that you keep in mind that these constants, in general, are wrong. For lower levels, they are adequate and fit for purpose. However, if you are playing with clearance considerations from the hard bits, the variation in pressure height calculation errors really need to be kept in mind.
A link to an explanation
Bob Tait's Aviation Theory School - Altimetry Rates - Bob Tait's Aviation Theory School Forums