A good rule of thumb is for every bird you wish to keep hatch 10. So if you're hatching and raising about 30, that should give you three solid birds to add to your breeding program. Glad you are keeping an eye on carcass quality, not many folks these days actually eat their Langshans, which is an important aspect of the breed in my view.
Sounds like you've got a good handle on space. Langshans do well on range (although I don't, too much risk, more so biosecurity from wild birds than predators though).
Historical traits are nice, but why limit ourselves to what they could do centuries ago? We know so much more about genetics, food, management etc. However, many breeds fall short in this way, they're either bred only for production and lose the look, or bred for look and lose the production. Will get off that tangent though, I just think we should look to at least match, but SHOULD be able to exceed what was done centuries ago.
When you say size, do you mean weight? Height? Width? One thing that tends to go hand in hand with stilty legs is also knock kneed or "hockiness", which is something that's highly objectionable as well, so keep an eye on that from the front of the bird, selecting for nice wide bodies helps with this too.
Don't concern yourself too much with white tips on toe feathers, it's not a real point cut in the showroom and is something that is mentioned back in the earliest writings of the bird. It could be stress feathering too if it's a young bird, or juvenile plumage. Often my blacks have white tipping on foot feathering, or even wing feathers until they get around 4-5 months. Although now that I think about it the Blues don't, which strikes me as odd. Good plan to keep breedings small, I try and keep it to trios with the females being either full sisters or mother/daughter, just so that it's easier to know exactly what produces what.
Even after all these years I still don't know what to do with the breedings that produce superior birds than either parent. It's a nice thing to have happen, but until the offspring are proven producers of good birds themselves there's always the fear of regression. As poultry breeders know there is no such thing as maintaining quality, you're either getting better with each generation or getting worse. Generally though Langshans tend to be fairly easy to breed and reproduce fairly true to what you expect (to the point that I can tell which breeding trio a given cockerel or pullet came from due to it's physical traits without bothering to consult records).
