If you keep them in a pen and run situation, just feed them only what they can clean up in a day's time. If they leave any behind, feed less the next day. If the feeder looks licked clean by noon, then add more to their daily ration. You'll soon be able to gauge how much they can consume of it as they grow, as seasons and needs change, etc.
I always tell folks it's just like feeding your own kids when they were toddlers....how much do you dish out on their plates? What they will eat and no more and that changes as they grow, so make changes accordingly. If they are leaving food behind, you give them less next time. Finishing too early, add a little extra.
Thank you.