frames to shoot in NEF
bias ~20
shortest exposure at lowest iso with no light reaching the sensor

dark ~10
same exposure time, temperature and iso of the light frames
usually taken during the photo session

flat ~20
an evenly illuminated frame at the same iso as the light frames e.g. the twilight sky southeasat about 10 mins after sunset. make sure telescope is focussed, expose on A-priority and same orientation as the taken shots.