I just give them plenty of hay and if they have eaten it all by the morning then I try to add a bit more in, like is a stuffed toilet roll as there was this morning (I just had the hay rack and spare hayrack/muzzle yesterday stuffed with hay and they had eaten all out the hay rack so have put in two toilet rolls for them), I just gauge it by that.
I think the solidness of the poop can be change by the fresh foods not the hay, too much rich foods will make mine a little loose, the hay is always the food that they absolutely have to have which is why I make sure they just have more than enough.