Feeding chickens enough (but not too much)

I wrote that one of the advantages of free range chickens is that you can feed them much less...and the next day it seemed like our young pullets and cockerels started wanting more to eat.  I suspect the frosts that wiped out the last tomatoes also killed off some of their favorite foods, but regardless of the reason, I suddenly felt like I was starving the poor little things.  Time to increase their rations back up to recommended levels.

There's a fine line between feeding your chickens little enough that they want to forage and so little that you stunt their growth, and I think I might have stepped over the line briefly.  I won't really know until I pluck them and put them on the scales, but I figure if these broilers weigh less than the previous batches, it will be my fault.

Meanwhile, I'm slowly coming up with some helpful tricks.  There's the full crop technique I've mentioned previously, but as chickens get bigger, it's tougher to tell how full their crops are through all those feathers.  One idea is to decrease your chickens' rations by no more than 30% to 40% and watch to see how fast they eat the food up.  If there's any food left on the ground fifteen minutes after the morning feeding, you can probably cut back a bit --- they're getting plenty of forage.  On the other hand, if the food is gone in a heartbeat and the chickens start obsessing over clover (a moderate quality food), then they might not be getting enough to eat.

I feed our adult chickens only once a day, but I've found that it's easier to keep rigid control over broiler rations while not under-feeding them if I give the chickens a second snack at nightfall.  If the day has been cold and rainy and the chickens don't look as well nourished as usual, I can increase their dinner without impacting their urge to forage.

Do you have any tricks for feeding your chickens just enough storebought feed while making sure they forage as much as possible?