Thanks. It turns out my regulation was far too wide. When I tested it for the first time I thought "no way, there must be something wrong", but there wasn't.

I removed the part of the wedge that was extending in front of muzzles to avoid it having any influence. I made a piloted crown cutter and I recut the crowns (more info and pictures of that process on another forum I probably shouldn't mention by name). I even tried Daryl_S's muzzle. I had to shape it by hand, because my crown cutter was made for 45 degree crown and I didn't feel like making a new one after spending lots of time on the first one.

However, with regulation set to converge at 50m 24 grains seems to shoot close to parallel. 75 grains goes very wide (almost 4.5mm per meter!).

If one simply divides the separation by distance and subtracts resulting divergence from the barrel angle the result is that they should "cross" even more than Pedersoli set them to originally. At the same time I know it was shooting cross-eyed at original setting. This in my opinion proves it is a non-linear problem (at least with black powder, perhaps it is with nitro).

So after testing I unsoldered the barrels and I resoldered them almost the same as they were originally, but with one important exception. I didn't solder the muzzles together so it is possible to spread them apart by inserting shims.

Once I finish the screw contraption (as shown on the regulation picture done by pros) I'll be able to set them as desired.