Hey Rolf,
Man, you are making fast progress!
Re the position of the box, there are several ways to center a box. Various builders approached it differently. You might want to see some diagrams I posted a while back. These might help you as you think about what you are trying to represent. Here's the link. Scroll down and you will find the images.
http://americanlongrifles.org/forum/index.php?topic=24772.0The pencil sketch you showed for box position earlier shows something similar to the box position in RCA 100 (which is an unsigned rifle that Shumway attributes to Beck), where the box is more or less centered where it intersects with the butt plate, but is otherwise parallel to the top of the comb.
The position you show with the paper pattern is, in contrast, more like RCA 39, which is centered along the axis. (Shumway suggests that RCA 39 is similar in some ways to a Beck, but you might note that box alignment in that one is different from the "Becks" in RCA 98, 99, 100, and 101.)
I don't mean to suggest that you need to follow any particular alignment. Just commenting about this because you mentioned Beck.
It's all a matter of personal preference, of course, and if you are building something of your own design you aren't locked into anything. (And personally speaking, I really like RCA 39!)
As for vertical centering, also keep in mind that some old builders centered the box between the toe and the top of the comb, whereas some centered it between the toe and the bottom of the butt plate extension. Some included the toe molding, while others excluded it, etc.
I found it helpful to make a small set of wooden dividers to use when comparing and plotting out what measurements compare to what on the photos in the books. (I hate to use metal ones on the books, as they scratch up the images.)
Whet