I was doing this for myself as I have no idea about how this breed grows and when would be the best time to harvest them. I figured a growth chart would be helpful and a resource to look back on later. If it helps others, then there are pluses to all of it. Then other can try and teach me something also.
I have the two roosters I like size wise, both are just short of 5 pounds now. 1 is 1 oz off and the other 3 oz off. They are gaining 4.5 - 5oz a week. The other two roos are 11 - 12 oz off, but the one is showing pretty tail feathers. I don't know anything either on judging them by body style and appearance. The two big rooster started getting tank looking last week and one of the smaller this week is tanking. The other small rooster look gawky compared to them and long boned basically. So I am not sure if he's a late bloomer and could get bigger than all of them, or just stay skinny. A couple weeks more will tell. Ideal hatchery lists the Roos at 10# and hens at 7.5# but no idea on how long to get there. They could be tough by then. So far at 17 weeks and I am half way there on their size of the birds.
I have 4 maybe 5 nice hens by weight but waiting to see how they all progress. Most are black with brown mottling in the feathers and one or two showing more brown. It seems to be they are the odd ones, but one of the brown is the bigger hens. So I don't know which ones to cull and which ones to keep. The wife says keep them all, she likes watching them.

She said if I grow any for meat they can't leave the pen and free range. Then she likes them. The biggest rooster 1786 has a group of hens already and forages with them near the house. making him more special.
I wish I could find more about dark cornish to read, but anything cornish on a search engine is dominated by cornish cross and you can't find anything else.

I feel like I am dwelling in my ignorance of the breed and just learning as I go here.