How much grain should a heifer get that is going to calve soon?

It's not advisable to grain a heifer that is ready to calve soon. The fetus at late gestation is focused on growing, so giving extra energy and protein to a springing heifer could encourage calving problems once the time comes for the heifer to give birth, no matter how thin the heifer is. You should leave off the grain until after the heifer has calved, and put her on a pound for the first few days then raise it up gradually over the coming two weeks until you're feeding her around 1% of her body weight as-fed per day. This is especially important if she's been on forage and hasn't had grain for a while, because you need to allow her rumen microflora time to adjust to the change in diet. If the 1% doesn't seem like it's doing much, then up it to 1.5% of her body weight. Usually feeding grain at 1 to 2% body weight is sufficient to get an animal on the road to gaining weight again.