I have two Buff Orpington hens that have been laying for two months. I feed them 50% (by weight) Purina Layena well mixed with 50% Purina Flock Raiser. I track their feed consumption very carefully....they are free-choice fed and consume 0.25 pounds per hen per day. They are also free-choice fed oyster shells and granite grit (layer size). They are not free-ranged. Under this diet, my two hens are averaging 13.5 eggs per week and one of the hens has laid 32 (to date) consecutive daily eggs. I hope that this helps you in feed planning.
With respect to oyster shells and grit... It is my understanding that oyster shells are consumed for the calcium balance in their diet....being digested quickly for that purpose, and should not be considered grit. Granite Grit moves from the crop to the gizzard, stays in the gizzard and with muscular contractions of the gizzard grinds up the food for proper digestion (to make up for the lack of teeth in chickens). The Granite Grit also gets slowly ground up in the process of muscular contractions of the gizzard and as the grit becomes smaller, get processed with the digested food, out of the intestine track. I have heard of a 15 to 20% reduction in feed consumption with proper grit sizing over not being fed grit...so as I see it, grit is important.