Mine have got whey from home produced cheese, and I used it to soak sugar beet shreds and then maybe reduced pellets by one scoop. I have regarded it as a treat, as it wasn't regular enough to think about by how much I should reduce the pellets.... they love it though, but the occasional milk from the goats (when our milk "lake" is getting too large for my cheese production line) is even more appreciated, again soaking shreds in it.
If you have a regular supply you maybe should do some calculations as by how much you can reduce the pellets, especially if you use it to soak other stuff in.