Carrie,
I had a skinny, super athletic english pointer that normally would weigh about 50#. During the blasto treatment he was down below 35#. Nothing but skin and bones. He looked terrible and just wouldn't eat.
Kinghty made it, it took him quite a while to gain back his strength and wind, but afterwords he was good as new, if a little spoiled.
Only you and your vet know how much is too much, but do keep in mind that many of us have been down that road and are here to tell you that this is beatable. We've all thought the thoughts you're having.
Good luck, let us know how things are working out for you.