The big question is how big are the bales/flakes? A flake out of a 35-40 lb bale would weigh different than a 60 or 80 lb bale. Once you have that information it will be much easier to determine if that is the proper amount. 6 flakes out of a 35-40 lb bale would be fine for a easy keeper. That is what my haflingers get in the winter, but I doubt that would be enough for a hard keeper. On the other hand, 6 flakes out of a 60 lb bale would be way too much for an easy keeper but adequate for a hard keeper.
You could also supplement with a "complete feed" that has enough fiber in there to count as a forage or beet pulp during the winter.