wow guys, lot of thoughts here! ok, here are some answers:
1) I've looked at the original pic and it is a bit deceiving, so I took them again:pic1, pic2(maybe again not 100%)
2) To take out any misplacement of the nut, I made sure the ruler matched the 1st fret, but again the position of both the nut as the 1st frets align. Just to be formal, I'm pretty sure the ruler was used correctly (so starting at the end of the nut the nearest with the 1st fret)
3) there were never any modifications made on this guitar (no Buzz Feiten)
4) when performing the calculation suggested by lovekraft, I measure between center 1st and center 13th 306 mm, so the calculation result=648,39mm
5) interesting remark of Gorecki which got me thinking, there was no radius whatsoever first, and now got myself a nice 9" radius on top (made a cylindrical board), so I placed a few times the ruler both squared/center, as in line with the positions of the strings, and turned out the result remains the same
6) I will make today some cardboards for in-between 25.5" and 26", and assemble the neck and body, and see if I find a matching "in-between" scale, so I don't have to do anything -except refret- with the frets.. keep you posted