Francesco,

That is three stops more exposure then a reflected meter reading on sunlit snow after allowance for the filter factor.

If I were making an exposure in "flat" lighting due to overcast sky or early or late light I would not filter the exposure and I would place my snow values at Zone VII and make my exposure but I would then give the film N+1 development. The filter would not be effective under flat lighting since the blue reflections within the snow are caused by the blue sky.

Good luck, I anticipate seeing the results of your efforts.