Hmmm, my suspicion is that you should breed from the ones with the best type and best lacing.
A excellent old breeder in the US once said to me 'go where you are pushed'. What that means is set your goals and select the breeders that match that goal. By all means inform your decisions with the best knowledge about genetics but don't be afraid to go with where your targets push rather than what the conventional wisdom might preach.
Totally agree with this as it make perfect sense in most cases but i'm finding the lack of consistency in the base gene is directly related to the lack of consistency in the offspring and therefore i have started culling to refine to eb down pattern as these are the birds that hold good lacing better. Red takes longer to develop in the growers though when compared to a eb/eWh sibling so it feels a bit counter productive at first but they often end up just as dark if you give them the chance. Many of the eb offspring from my line are also a bit too melanized (heavy lacing and black hackles).
My theory on why things are the way they are in this line is because an approach where good red was put at the top of the priority list was used coupled with an approach similar the quoted text. Since the best red birds are often eb/eWh and some of them will actually hold lacing, many split based birds have been retained over the years and extra melanizers have been selected for in an attempt to get the lacing to hold. It works but not consistently. Of course when you breed eb/eWh to eb/eWh you get 25% eb and 25% eWh as well as 50% splits. The full eWh almost always have no lacing on their fronts so end up culls. The eb's don't develop red as early as others so could potentially be culled if you were hell bent on the best red (not to mention they are often over melanized to take time to clean up). This leaves you in a repetitive cycle of inconsistency. My approach has been to weed out wheaten and refine to eb base then start working toward consistency of red as well as red/black balance. To add a little more to the story, since so many generations of splits have been bred together and the ones with extra melanizers to achieve good lacing are chosen as breeders, when you use these birds and retain the eb's from them, they are often over melanized.
As mentioned, the red takes longer to develop on eb and any weakness in the red is very obvious. This, i believe, is a blessing in disguise. Since the weakness is exposed then only the best genetics for red ground colour get fostered. In time i believe this will lead to much better consistency (and eventually quality) in the red ground colour.
I am not telling you how it is, merely expressing my findings and theories in an effort to help. I am confident enough that i have culled more chicks than i'd like to admit over this season as it was getting to a point where i was producing far too many wheaten carrying chicks and it was becoming too costly to grow them all out only to find the lacing washed out as they approached their adult moult. I can't afford to be spending too much on feed for birds that have no place in the market and therefore need to be culled so this seemed like the smartest approach to me. A few others have taken this approach too.
Getting back to birchen though, i hatched one chick recently much like you described. Birchen looking down pattern with dusky legs. I've kept it out of curiosity and it's still young but not looking too bad. I'll have to dig up a pic and remember how to resize them (it's been a while
PS Would be keen for an egg swap in spring if you're interested. Have just started mucking around with bantams too.