The corrections for temperature variation from ISA need to be applied.
If the airport was at sea level, the assumed temperature would be +15C.
The temperature is assumed to drop at a rate of 2C/1000 feet as you know, but from the origin at sea level, so at 1000 ft the assumed temperature should be 13C giving an ISA temperature difference of 17-13 = 4C.
Make that correction and then the rest of your calculation s are correct.
