Or you can do what farmers, exhibitors, and breeders do... keep the top 10% in growth rates, vigor, production, health, etc, and cull the rest. In every flock you will have a favorite hen. If you start with 35 and only keep 3-6 of your favorites then you will have a better flock.
I would start by looking at the growth rates around 8-12 weeks and anyone that is well under the average for their breed would get a black legband to be sold. At that age I also would sell anyone else that has imperfection that you don't want to keep in your flock. If you sell 30-60% then you won't run out of room as they grow. The rest of the pullets would all be laying by March or April. You can then start to figure out who your good layers are and who the slacker are.
The Hogan method,
Culling by Head points, and
flock culling (
Ref 2 ,
Ref 3) all can help determine the good layers if you can't spread the pullets out to different pens or trap nest. The Hogan method measured the distance from the end of the keel to the pelvic bones, the width of the pelvic bones, and the conditioning of the bird judged by amount of meat on the keel to determine who are the top layers. The Culling by Head Points method looks for birds with a correct heads to determine their vigor and laying potential (i.e. eyes in the front 50% of the comb, head being more flat from side to side than rounded, the distance from the ear lobes to the beak being well proportioned to the distance from the eye to the comb, etc.). The flock culling involves checking the vents of all the hens in the flock every week as they come into lay to see who starts to lay when. Color depletes from the laying hen so there is also clues from that that can determine how long a hens has been laying. Then starting in July you vent check the flock once a month. Anyone that stops laying in July is a cull. Anyone that is not laying in August is not a good contributor to the flock either. Your hen that are poor layers typically stop laying in September. Your hens that are good layers usually stop laying in October. Only your top layers will typically make it to November before they stop laying for the winter molt. In October you can cut the flock down 3-4 of your favorite hens then you will have the coop space to win another Greenfire Farms auction and start the process all over again. In a few years you will have a flock of 6-10 of your favorite hens.
Farms cull their flock because the flock is not profitable if they keep hens that are not good layers. Exhibitors cull their flock because not every bird is a champion and they are after the very best examples of the breed to show as ambassadors for the breed. Breeders cull their flock because pure bred flocks involves a lot of inbreeding and if they don't cull the weakest birds and focus on breeding the strongest inbreeding depression will lead to a reduction in size, reduction in health, reduction in production, and reduction in fertility.