Yes I am doing differential photometry and relying on the standard comparison stars.
To summarise – avoid median – non linear operation for stacking and this leads to exposures which should be recorded as the sum of the exposures in the stack. Because of the way I take the frames I will have the equally spaced exposures of equal exposure times the mid point of the central frame will be very close to the stacked observation time.
Thank you for your advice – newbies can get a bit lost on some of the details. Last year’s data was processed with Muniwin which did not require much in the way of decisions about stacking – however the down side was entering the locations of comparison stars, I felt this was very error prone. Richard Lee’s spreadsheet for processing AstroimageJ measurements file does this in a very slick way, hence my experiments with this approach. However stacking raised these issues which you have answered. Thanks again