Here's a pretty good online ballisitic calculator. You can manipulate elevation and zero distances amony many other things.
http://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgiAside from atmospheric pressure, Another factor going from 3000 feet, up to 12,000 feet in elevation may be your muzzle velocity. MV may be lowered a signinficant amount by the drop in powder temperature.
Using Reloder 22, and Reloder 17, I see approximately 1 fps loss in MV per degree Fahrenheit.