'optimal' ? can have different meanings to several people.
but.... and there's always a but.....
if the snow's dry, you can use less RPMs and save on fuel.
kinda like asking 'optimal' ground speed ?
one could say 'go as fast as you can, but as slow as you must'...
really there's a LOT of factors and since YOU are there, running the machine , YOU know best. IF 2500 gets the job done you ARE saving fuel from running at 3200.