I guess you missed the sticky here:
Altitude vs. Barometric pressure
Altitude, including pressure altitude, density altitude, etc, are not external ballistic parameters.
For local air density you need:
For your bullet BC input you need to select the atmosphere standard that qualifies your rated BC(like StdMetro, ICAO) as a baseline. These terms are again:
Much of the other stuff people come up with amounts to misunderstanding of these things.