I used to weigh the chickens here on a regular basis, especially the broodies.
I was talking about it with the guy I know who keeps game fowl. I was concerned because Mini Minx had lost almost a quarter of her body weight since I last weighed her and she was sitting. She got up every day and did all the right things, ate like a horse.
My friend asked when did I weigh her. I told him the date. He said "no, no when did you weigh her." We have language problems needless to say. He speaks a very strange combination of Spanish and Morrocan and I speak Catalan and English.

Eventually I came to understand what he was asking was at what point in her egg cycle had I weighed her. Well, now the penny dropped as they say and it didn't take much of a leap in intelligence to realise that if you weigh a laying hen immediately after she has laid her egg you get her true body weight. Onn the other hand, if you weigh her just before she lays an egg you get her body weight and the weight of the egg! Sitting hens won't be making eggs so any comparison unless the weight is taken just after egg laying is likely to cause alarm.
It's another one of those pearls of wisdom one doesn't see mentioned that experienced chicken keepers know about. Not quite sure what that says about some here on BYC.

So, weigh your hens straight after they've laid an egg. That will give you their true body weight.
Very roughly, between one third and one quarter of a hens food intake goes to make an egg. If a hen eats the same amount of food when she is not laying she should in theory put on weight. After all, she isn't rushing about burning off calories while she sits. So, if you measure the amount of feed a hen eats while she is laying and then measure the amount she eats while sitting you have a reasonable indication of whether she is eating enough. My game fowl friend weighs the poop. His hens are confined on their own, or with their rooster so this is easy.
The best sitters here, as in most feral and independent do a double eat. They get off their nests and head straight for the nearest food source. They eat first, then broody poop, then dust bath, poop a bit more, chat with their mates and forage.
I got told to leave feed down right until the hen gets back on the nest because Donk for example will go back to the feed point after foraging and fill up on the commercial feed. It's the second feed that seems to be the key here.