I'd just give her an appropriate sized rat weekly and feed in the enclosure. If that's a medium based on her weight and size, feed her mediums. She will get there once she is on a normal feeding schedule with proper husbandry. You should see some of the animals posted here that are rescued in horrible shape. Once they are properly fed and watered they bounce back quite nicely. Resist the urge to overfeed just to get her to the weight she should be.