I have never heard of chickens being overfed before. I think they eat until they have had enough. (I have read cautions about feeding too many treats, because they will fill up on those and not necessarily get the necessary nutrition the 'boring' chicken food gives them.)
From all I've read here, the amount of food a chicken eats depends on breed, age, sex and possibly even time of year.
I would do my own experimenting for the amount of food that seems right for them - and make adjustments as needed. So if they seem hungry, give them more food. When there is some food left in their feeder, you know you have reached the amount that is right for them.