[Shiraz (Ray)]

All that is needed is enough sky signal so that the associated shot noise overwhelms the read noise. The read noise of this camera is so low that you don't need much sky noise to do that.

if you use high gain, the read noise is lower and you can use shorter subs to overcome that lower read noise. High gain with appropriate short subs will give exactly the same SNR results as fewer, low gain, longer subs, provided the total exposure time is the same. It is counter-intuitive, but the best sub exposure will produce subs that look pretty crappy individually, but that is not the final goal - take no notice of what the subs look like - just the final stack. If you insist that you must have subs that look good individually, use longer subs and accept that you will lose some dynamic range.

If the sky is bright, the sky noise will overwhelm the read noise with shorter subs. It isn't that the short subs give you better results than longer ones - it's just that they do not make the bad situation (high sky noise) noticeably worse.

So in summary -
high gain > use more, shorter subs
bright sky > you can choose to use more, shorter subs

I have tried 30 second and 1 minute exposures with my f4 system under fairly dark sky - a final stack of 15x1 minute subs was slightly better than that of 30x30seconds, as suggested by the table, but the difference was not extreme. Glen, with your f5 system at gain 50 under mag20.5 sky, I would be using maybe 1 minute rather than the 35 seconds (which is recommended for a significantly brighter sky).

Since the signal is low in the subs, floating point stacking and processing is required - if the stack or process software has fixed point internal representation, there might be a loss of signal resolution leading to poor outcomes. I use Nebulosity and PixInsight for all processing - these are floating point. Anyone had experience with other stack or processing packages? Photoshop for example seems to be 16 bit fixed for most operations, but I really don't know much about it.