Well, she really doesn't look to be underweight to me. It does appear that a small rat would be a big meal for her. I would want to weigh her and get a good idea of her exact size and then weigh the meal that I plan to offer her. The 10- 15% formula would then apply. Her overall appearance in the photo looks very good. Her scales look shiny and hydrated and her body appears firm and muscular to me.