Start out with the same volume of food you give dry. Generally that's what they'll eat. Mine take a bit of watching because for some reason they eat like pigs some days, and some days they don't.Making a wet paste out of it and giving it to them in a bowl cuts out the waste. Downside of this is that you can't just fill up the feeder and forget it. You have to feed them once or twice a day this way. But it does cut the waste almost all the way to zero and if you have wild birds getting in the food, they won't get in the wet food.
Well, I already go out there before work, after work to let them free range, then before bed to close their coop after they've settled onto their roosts for the night. I open their feeders between morning and coming home from work then I close them the rest of the night. So it wouldn't be an issue to feed them with paste. Thanks for the suggestion. How do I know how much to give 15 chickens of the paste though?