Satin Balls- weight gain for surgery?

My Tilly is approx 5#. The vet mentioned at her last check up that she is a bit under weight. She just doesn’t eat well and she’s extremely active.
I have decided to have her spayed and really want to put a little weight on her before the surgery.
A friend recommended the Satin Balls. I was concerned that Tilly wouldn’t eat it, because she’s very picky. But I made a batch and gave her a bit to test. She ate it and came back for more.
My question is, how much do I feed her?
Tilly doesn’t typically eat “breakfast”. It’s offered. But she doesn’t usually eat. She does get several treats before I leave for work, several more treats at lunch and then dinner.
So my plan is to continue with her regular meal in the evening and give her a satin ball for breakfast instead of offering the kibble she doesn’t eat.
I made the small batch, with 1# 73/27% ground beef. The instructions said to divide that into 14 patties. But even that seems like a lot for such a small dog?