Great topic.
I have been wondering about the same. Most people seem to stick with 'standard' subs of say 10mins etc, but surely that's quite random and cannot be optimal. Given that time is our most precious resource in this hobby the goal must then be to capture the highest S/N in the shortest amount of time.
I did standard 10min exposures until doing those exact calculations for my setup recently. According to the calculations my ideal sub lengths turned out to be a lot shorter. Being still somewhat sceptical I settled on a compromise of 5min subs for my
recent image of Barnards Galaxy. That reached a limiting magnitude of ~24 which is a good deal beyond what I have previously managed, so can't say I'm disappointed.
Currently I'm working on another deep image with even shorter subs, 2-3mins, and the data looks very promising so far.
Something to also bear in mind is that ideal sub lengths are not the same in each filter (L, R, G, B). So I now use four different sub lengths and RGB subs range from 2 to 6.5mins.
BTW, I use scaled darks so no need for different dark sub lengths and that has served me very well so far.