I dont think you have to stress out about it so much at her age, she will be much bigger before you realise. Just give her what she will take and looks healthy for her. This isnt about the math realy .

I do sugest switching her over to rats, while she is young and doesnt care about what she is eating. Fedding 5+ mice plus to a juvy/adult BP must be harsh... a Rat pup should be the same weight as a adult mice, 25~g around here, and if a pet store has them, they usualy have bigger sizes too, so you dont have to buy 2.