Ray,
I have just run my current system through your spreadsheet with great interest, as I had done extensive work to dtermine optimum sub times for LRGB. System is: ASA10N at f/3.6; Moravian G3-16200; ASA DDM 85A unguided mount.
My theoretical work came up with 5 min as a safe optimum - and that any longer would be overkill. I determined it was better to do more subs to reduce calibration noise. (and dithering is very important in this)
Lo and behold, your spreadsheet comes up with the same answers that I got: above 3 min is generally OK, and 5 min is safe - ie on the flat portion of the S/N curve.
So from this initial look, I'm sold
With many thanks,
Mark