Very sophisticated metalwork, and I especially enjoy the drawing even if I can't read millimeters.
As for the rear open sight placement, when I worked for Green River Forge building muzzleloaders for guys who were then my current age, the standard was about 25" from the buttplate. I think you will find this working quite well for serious open sight shooting.
I wrote two columns about my thought on iron sights for primary and auxiliary use in recent issues of Sports Afield.