I'm new to the hobby, and also have leucomelas froglets (popular starter frog, eh?)
I don't know how you folks count fruit flies, or if you've just developed an eye for approximation, but I gauge the next quantity of food on what happened to the last quantity...
If there are tons of flies crawling around the next day, don't feed.
If there are a few random flies hidden here and there, feed the same amount.
If there are no flies left and the frogs are actively hunting springtails, feed more.
I enjoy watching the hunting behavior, and will likely shoot for this when the frogs mature, but right now I want my babies to get fat and grow up!