I think there will only be a very marginal difference between the amount of flush volume needed for smaller nozzle sizes as most of the internal size of the nozzle between filament cut off point and tip of the nozzle will be the same size - it will just be a little bit at the end of the nozzle that is thinner.
In terms of pile up of flush - it think it will certainly be taller for smaller nozzle sizes - as the width of each stand will be less - so more turns will be required to achieve the same volume.
I think you could probably reduce the multiplier - but if you do need to flush a large amount then it might be worth looking at the enhanced filament change gcode developed by @AMT_MW here - as it has a feature to separate large flush pile ups into separate parts chunks. - see this thread Multiple poops with high flush volume please - #45 by Cenwulf