Remember, you need to increase idle rpm as altitude increases to keep combustion at efficient levels. The compressor needs to turn faster in the thin air to deliver the minimum pressure required for combustion.
Full size jet engines do this automatically through the barometeric function of the engine control system , hydro mechanical. the BFCU, or electronic. Result is idle rpm will be very significantly higher to keep the compressor delivering the required airflow due lower density to keep combustion working sufficiently well to keep the engine running!
As a rough guess I would suggest raising the idle rpm by 2% per 1000 feet of altitude.