Rolf, try tracing the blueprint out on paper first and then redrawing the upper line of the wrist to get the proper curvature, possibly making the wrist both taper a bit more and maybe a bit thicker. You may then need to adjust the comb nose to get that right, and so on.
You probably already know all this:
A few months ago I had to draw up plans for the current build, using a longer pull and significantly smaller barrel, while trying to preserve the overall architecture of a rather short-coupled, big-breeched original. I pretty quickly discovered that trying to use any of the actual dimensions of the original was a mistake. The trick was to look at it in turns of proportion - the wrist tapered this much, the comb was so high compared to the depth of the wrist at the same spot, (importantly) the buttplate was so many times the height of the wrist just behind the breech, and so on. Of course, my final design didn't use exactly the same proportions as the original either, but by looking at proportion and overall shape and adjusting as needed I think I kept something of the overall flavor of the piece.
When I was done, I went ahead and made a kind of half-model (flat on one side, rounded on the other) out of a piece of pine lumber to make sure it worked in 3-D. You might consider doing the same thing.