We've had extremely high success rates with out flock over the winter. We selectively cull for hens that insist on "holding out" in the winter light or not. Couple generations later we're getting typically 30-35 eggs a day between 50-60 hens. Of course we also have broody hens year round as well.
Anyway, back to the topic. Free choice feed for one week and monitor consumption. With that many hens (3 right?) I wouldn't suggest leaving the container full 24/7. Rodents, spilled feed, mold, stale feed can occur. I would monitor the intake and then add enough for their daily ration and a little extra to carry over. The rest you can store inside someplace covered and dry. I also would suggest looking into pellets or ground corn. They can be so wasteful with crumbles. We have a large flock year round and every ounce adds up quick. We do a custom mix. We take 16% layer mix, 18% layer mix, ground corn, oats, and chick grower for extra fat and mix it all up. Big plump healthy birds.