There are so many different ways to manage a flock for production that I can't really say which is the "best", but I've found the following to be workable.
I get chicks in the spring or early summer. They start laying their first autumn, when they're around 6 months of age, and continue to lay for about 12 months, until their second autumn. At that time they're around 18 months of age, and typically go into a fall molt, taking a 2-3 month period off from laying. Unless the hen is a very poor layer, I keep her until the fall of her 3rd year, at which time she will be around 2 1/2 years old. I then cull these hens in the fall, unless an individual hen is a fantastic layer. (I only have a dozen hens and can track production for each individual, which is not possible in your case.)
Some people routinely cull during the second autumn, when their hens are around 18 months old. However, if you keep good egg laying breeds, I really think it's worth while to keep them until their third autumn, and then cull them when they begin to go into a molt.
With regard to feed, I've been told by Extension poultry experts that birds tend to waste about 25% of their feed when mash is used. I switched from crumbles to pellets 2 years ago, and noticed there is much less waste and less feed consumption (= less cost).
I use Purina flock raiser (20% protein), and get very good egg production from my hens. Better than when I was feeding them a layer formulation (16% protein). Some of my hens even continue to lay while molting, which didn't happen with layer feed. I don't know how well leghorns would respond, but my hens (RIR, barred rock, dominique, australorp, EE) have done well on it.
Edited by song of joy - 10/19/15 at 10:42am