Temperature error is about 4% per 10c above/below ISA of the air layer beneath the aircraft.
In your example the OAT is -20, ISA at 10000ft should be -5 so the air mass is ISA minus 15 (assuming linear increase, airfield temp -5, MSL -0)
Indicating 10000ft, actual height AMSL 9400ft so your height above the airfield would be 6900ft.
If the air mass is 15c warmer than ISA your actual height would be similarly higher (+600ft) but that is not a problem as the Cumulo Granite is further away !
Computer not required !
