I judge based on weight and health of the mice. If I'm not feeding enough or often enough, the mice lose weight, and if I'm feeding too much, there will be lots of leftover bits in the litter that went un-eaten.
A good way to judge feeding amounts, and nutrition of your food, is to look at your litters.
Do your babies look chubby and healthy? Or skinny and frail?
I just noticed this, but I would recommend moving the hopper away from the side of the tub, because the hopper rests near a little lip and the ASF's will have greater chance to chew on it. I'm just warning you, as I had an escape from something similar.