I made the following charts for the air data lab but you will find them most useful to show the relation of simple pressure to the atmospheric altitude. Note the rapid drop off of pressure. The sea-level pressure of 29.921 inches of mercury drops to half at 18000 (which is why we can't breath without oxygen there) and to only 10% at the highest range of aircraft ops. Use this chart to determine the relative change of pressure resulting from any two different alitutudes. Examples. Sealevel to 3000 feet is 5 in Hg, but 20 to 30 thousand feet is also 5 in Hg.