Ration feeding, or portion control feeding is a whole lot more art than science. The science says 5-6 ounces per laying bird, larger fowl. OK, but the variables differ, as you say. This is where the art of husbandry comes in. Do they need to eat just a smaller supplement because they are successfully ranging? How rich is the range, based on time of year? Is the ground snow covered, or made virtually barren by drought? How much more will a hen take keeping herself warm during the cold winter months?
I portion feed. It just takes experience, observation and flock awareness. I feed both morning and late afternoon, If there is feed remaining, lots of it, as they go to roost, I've obviously over-portioned and I'm just leaving feed out all night for the rodents. How much of the morning's portion is still in the feeders before I add to it in the afternoon?
Portion feeding is fussy. It takes focus and observation. It isn't for everyone.