If she is skinny, you should feed her more.
Really though, you should weigh them. If she is 55 lbs, she would get approximately 2% of her weight (it varies, but this is about average) so a little over one lb per day (not per meal). If you want her to gain wait, you can start by adding about a quarter of a lb more per day, see how that works, and if she's not putting on the weight after a week or so, add another quarter of a lb. You could add more meat to her ration like ground green tripe, pork, chicken breast, heart, a whole egg, whatever you can get your hands on that's in your price range. And don't forget to add some liver on occasion -- they really need it. You can also add some coconut oil -- slowly -- for extra calories if she needs it but cannot eat much in one meal.
I don't weigh the food anymore, but when I did it gave me a way to be able to eyeball "so, this is what a lb of this looks like", etc. Now I feed less if they look chunky and more if they look thin or the same if they are just right.