A key thing is sub length and how many you shoot. With a C8 RASA, F2, in Bortle 7, I shoot hundreds of subs at 10-20 seconds. Example, 662X10". I've yet to find someone willing to suggest how often I should dither.
I'm thinking dithers per X time might be a more useful measure. If someone dithers every frame at 120", maybe I should dither every 12 frames?
This isn't really a suggestion so much as it is a mulling over of an idea, but I think, for example comparing f/4 to f/2 you would take subs a quarter the length for each sub to be equally exposed, and you'd need to take the same number of subs as the f/4 system (requiring a quarter of the total integration time) to reach the same SNR in the stack.
So with the same number of subs, you'd need to dither the same number of times to get the same suppression of pattern noise. You'd be dithering 4x as often going by the clock, but at the same frequency going by the number of subs.
Thinking about two other cases, say you decided to keep sub lengths the same (maybe not if hitting saturation, but perhaps for narrowband you might) so you got deeper subs with the f/2 system, you'd only need a quarter as many of them for the same total integration time so then I think you might not be able to dither enough - the f/4 system might have 60 5 minute subs in 5 hours with a dither every frame, and you'd have 15 5 minute subs in 1.25 hours and couldn't dither more than 15 times.
Conversely assume you kept the per-sub ADU count similar by exposing a quarter as long per sub, but just used the whole 5 hours and got a deeper total integration, now you'd have 240 subs of 1.25 minutes each, and I think then you could dither every 4 subs to have the same total dithers in the stack (and same dithers per hour of clock time) with no increase in pattern noise compared with the f/4 stack.
Again, nothing particularly scientific about that, but something to think about and possibly refute or tell me why it ain't so.