A couple of the breeders around here use 18-20% feed, but admit that it takes longer to grow out the birds. Personally, my limiting factor is space, so I'll stick with 28% from hatching to hatchet. I also know how demanding it is on the girls to lay an egg every day, so I think that 28% for layers is warranted, with supplementary calcium.
Over the past 3 years I've averaged about 275 eggs per year with heritage chickens; about 75% laying. That seems to be a fair bit higher than is expected, but is likely a result of quality feed and good management (if I can be so bold). I care for my animals as best I can and they've always treated me very well in return with excellent yield and excellent quality. I honestly believe that they eat as well as I do and my layers give me eggs that take a full minute longer to hard-boil than the same sized eggs from the store. I think the difference is due to higher nutrient density, as my eggshells aren't that much thicker than the store eggs and the whites aren't as watery. Quality feed results in quality product, and I can easily justify the cost for a far superior product for my family.
Get the best feed you can for your birds and you'll reap the benefits.