Digital kitchen scales are pretty cheap at Walmart ($15). And adult mouse is 1oz-ish, so it would be one mouse for every 10oz of snake. I turn mine on with a basket sitting on it, then set Malfoy in.
If if she is acting hungry, I would offer another mouse or two at 5 days.
Granted, I have never dealt with a baby ball, but I would think if she is hungry, she should eat.