Rumble,
It changes from the temp, humidity, and baro readings and from elevation. The correction factor comes from the amount of water grains in the air by %. More water more o2 less water less o2.
Great info on your build,i like it a lot,,lol..
Its been a while but doesn't more water in the air displace 02 and less moisture make a denser charge..High humitity = a high density altitude,,wayne