Hi there,
I used grapefruit seed extract when my son had thrush (and I had to use it too b/c I was still nursing), however he was a little older and I was able to put it in juice... I don't know if formula would mask the bitterness of it. My bottle says adults 5-15 drops 1-3 times daily and for kids 1-3 drops , 1-2 times daily. It tends to settle at the bottom of the liquid if you don't just "chug it," so I would recommend using 1 drop at first in a small bottle first (formula I assume) to see how she does with that, then maybe increase the drops but keep the same amount of formula so she doesn't get all the bitterness in the bottom.
Hope it works for you as well as it did for us! It was cleared up in a few short days, if I remember correctly.