And that's a great "rule of thumb" --- BUT --- all hay is not the same. 10# of 9% protein hay is a lot different than 10# of 15% protein, or 18%. The QUALITY of the hay makes a big difference. You can have cattle bellie deep in grass or hay, and they can litterally starve to death if it doesn't have the food value they need. The poorer the hay, the more they NEED to eat, but usually cannot consume enough - due to the limited internal space.
So, if your hay was put up when it was still growing & didn't get rained on, and was properly dried they won't need as much as poorly put up hay.
Generally, I feed what they can clean up in a day. But, I (this will open a can of worms) believe in feeding weaned calves GRAIN. Very difficult for calves to consume enough hay (unless it is really good alfalfa hay) to grow properly.