Excellent Chris.
From your Bortle estimation and the LP map, your sky is probably a little darker than mag20. That means that, if the table assumptions are OK,
you should get a bit less sky signal than would be predicted by the mag20 table.
Now the mag20 table, at gain 90 and f5, predicts that you should get to 370ADU (290ADU + bias) in 55 seconds. When you did your test, you got to 300ADU in 60 seconds (assume that includes bias?) -
which is a bit less than the sky signal predicted by the mag20 table.
ie, your sky should have produced a little less signal than that predicted by the mag20 table - and that is what you found when you measured it. It looks like the assumptions were not too bad and the tabular data is at least in the right ballpark
Thanks very much for doing that testing.
So, can somewhat more confidently suggest that you could drop your subs to around 1- 1.5 minutes if you wish, without losing any SNR/sensitivity - you will gain extra dynamic range and possibly better resolution by doing so. Or you could continue to use longer subs if you want a lower image processing load and are happy with the dynamic range and resolution - you could consider gain 0 for long subs.
very grateful. cheers Ray