The small rat was about the same size as her girth so I decided that the rat was for her. I got her at Petco so I don't know when she was born. I know for sure that she is underweight since I can feel her spinal cord(?)
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.
IIRC the Arctic Mice brand feeders run on the small size anyway, the small rats are usually right at 50 grams (the ones from my local supplier run 55-70 grams). Assuming you're buying your feeders at the pet shop, she should be able to handle it. I'd handle her minimally for three days to give her time to digest the meal if it was on the large side.
In your other thread you indicated that she was a bit dehydrated so if she's taking f/t then offering the feeders wet will help with that.