Is it ok to leave what's left over of the FF in their dish, to the next day? Have trouble determining how much they're going to eat so there's always some left over, do i have to scoop it out and put it back into the FF bucket? Or maybe just throw out what's left and start new each day?
It's fine. As long as it's not too cold {or hot enough to pasteurize}, the bugs should be ok. It's a midnight snack.