Don't forget the pressure altitude difference:
ISA=1013hPa
1hPa=30ft
For example:
QNH= 1031
=> QNH-ISA=Pressure difference
=> 1031-1013=18hPa
=> 18 X 30 (ft X hPa) = 540 ft
therefore at ground level on this kind of day the aircraft will perform like it's 540ft underground
=> increased performance.
Adding this to Greaser's case above (assuming QNH=1031 hPa)means that the density altitude corrected for both temp & pressure will be 5860ft instead of 6400ft.
Niall