I'm no chicken expert, but I noticed that nobody had answered your question, so I'll take a stab at it:
We feed commercial feed twice a day, with garden scraps in between. We also let ours free-range much of the day.
I usually gauge how much to feed in the evening by how fast they come for it. If they scramble to empty the feeders, they get more. If they are fairly indifferent, then I don't top the feeders off.
If your birds are confined, it's easier to estimate the feed levels. You can leave the feeders full if you're not going to have a problem with wild free-loaders, or with birds "billing out" and wasting a lot of feed. etc. Ours are happier to free range, though, so I don't keep the feeders full.
Since ours are out quite a bit, so I try to judge the evening feed level by their behavior (and trying to guess how many peanuts the kids have been feeding them by the shells lying about).