At the time I had no other option: Either 5" aft of CoG, or 1" or less, because of the wing main spar. And I did not want to do that, because of the earlier X1200: that thing had its main wheels close to CoG, and had a very strong tendency to buck its nose up at the slightest bump under the nosegear (even on paved surfaces) which continuously pushed the prop into the grass.
I knew the 5" was a bit far back, and I did expect problems with rotating, but because of no other option, the stance of 3,5 degrees positive was chosen. That would ensure a take-off at sufficient airspeed. And in all fairness, when it comes to take-off, I prefer not being able to rotate but the LG dictating take-off speed. Better that, than pulling hard to lift the plane off and immediately go into a stall.
Now there is this thing with CoG, that it is not in its final position yet, it could possibly move aft by up to an inch (not expected, but possible) so we are going to adapt the drawings once final CoG is known. Relocating that main spar.

Tomorrow I hope to learn more.

On a more positive note: I was able to actually and accurately measure static thrust, and it turned out way better than Pé Reivers' propcalc indicated.
@12K RPM I measured 1,95 kgf, while the calculator indicated only 1,55 kgf at that RPM.
That probably is because Pé's sheet takes the fuselage drag into account.

Modifying the muffler into a "pusher pipe" kept the plane a lot cleaner on the test-table, and also changed the exhaust note a bit, the tailpipe now blowing straight into the prop arc, but it makes choking a lot nastier when the muffler is hot...