Snow is supposed to be 2 stops higher under a reflected reading. It's supposed to fall on zone VII usually under overcast skys. I'm assuming that you want texture in your snow. Always meter your shadows to see where your highlights fall. All meters give an 18% grey reading. Since snow is NOT 18% gray, but white with texture, it reads 2 stop higher which falls on Zone VII. So you must open up 2 stops from your reflected reading of snow to make snow white with texture. But snow under bright sun might make snow fall on Zone IX lets say. You'll have to do an N-2 to pull snow that's blown out to zone IX back to zone VII.
Ansel Adams the negative is a great book to learn the Zone system. Correct me if I'm wrong.