Avaition uses actual altitude to figure percentages of power at the higher altitude, no links or reference materail avaible that I can link to or tell you the names of the books and authors, sorry. My old airplane, 1960 piper Comanche had a newer 260 hp Lycoming engine it that was rated at 260 HP at full throttle and maximum EGTs at sea level at instrument standard atomsphere, 59F, 29.92 BP at mean sea level with no humidity, that engine had 55 % power at full throttle at 13,300 feet above seal level max EGT with ISA, which is one inch of B.P. loss for every 1000 ft. of altitude above sea level and around three degrees F temperature loss for every 1000 ft. above sea level. Maybe you can come up with a math formula based off of that for a chart Avaition does have air densitity charts based on current barometric pressure at the field, observed altitude(actual feild altitude) and the outside air temps with no humidty used in figuring the density altitude I like to use the weather stations made for drag racing to read their versions of the current density altitudes