Thanks, yep the seeing was probably one of the best nights of the year
I usually try a fixed number of frames, using the smallest number I can get away with without increasing the noise levels. Usually 3000 frames (out 75000) is the minimum, even then the noise was minimal. So I tried 1000, 1500 and 2000 and even with 1000 frames the noise was very low so I processed further. Seems to have worked out OK here