Feeding calves through the winter

Hi, all. I live in Vermont so, naturally, the winters will be quite cold. I'm starting my small beef herd this winter, and have 4 heifer calves that I've bought -- two born in April, and two born in June (but staying with their mothers until September before being weaned). Come September, I will have all four grazing until late fall -- sometime in mid to late October.

So, my question is, what is a good balance of grain and hay to feed them to get from mid October through mid May? I presume it will be easier to tell come winter what their needs are when I can monitor their BCS, but am hoping to plan ahead so I can secure better rates on hay. My initial thought is to feed 20lb hay per cow per day, and 3lb grain per cow per day. I would start small and increase rations throughout the winter as they are growing -- i.e. 2 months of 15lb hay, 2lb grain/2 months of 20lb hay, 3lb grain/2 months of 25lb hay, 4lb grain. Then, come late May, they will be on pasture again throughout the summer. I am hoping to breed them in the late summer/early fall.

Do you think this is a reasonable feed plan? What would you recommend?

(As for breeds, if you think it matters -- they are two Herefords, and two Jersey/Devon crosses)