Farzad
Hello. Most people, I think, think about this question a bit differently. When trying to determine optimal sub exposures, it is considered a better idea to think about what the minimum exposure needs to be to differentiate signal from noise. Or, said another way, what is the minimum exposure needed to get your weakest signal off the 'noise floor' of the camera? In this way, you would have the best chance of capturing the faintest details with the smallest possible exposure - thus giving you the least chance of over-exposing anything in the image. Stars will probably still be clipped, but there should not be any clipping in the nebula areas. Of course, there are outliers like M42 that may require an HDR approach (using short and long exposures) to avoid saturating the nebula, but in almost all cases, this approach should work well. There is a script in PI, CalculateSkyLimitedExposure, that can help you determine your minimum possible exposure while still getting all the details above the noise.
I'm sure others with more experience will chime in, but I hope this helps.
Mike
