Thanks. I got a fully automated stitch as described below, but I don't understand why.
About your questions: It is one single row with 18 pics (not multirow). I did a guesstimate at npp thinking parallax issues would be limited with objects mostly far away and many images next to each other in the horizontal plane, each 20 degrees apart (360/18). This is guessing on my behalf.
In any case I tested a random number in hor fov (I think 30 instead of 50) and that did the trick for the stitch in pw6. But the logic behind what value to put here is beyond me and I'm hoping someone can explain this.
The lens data (Sigma 28-70mm F2.8 EX DG lens) states angle of view 75.4 - 34.3. The lens was "zoomed out" so I'm guessing that gives 34.3 for hor fov? However, I had placed the camera body vertically on the rotator and perhaps this would give yet another hor fov value identical to the vertical fov of the lens unless pw6 considers the "long" side of the picture to be "horizontal" regardless of me rotating the camera body? I couldn't find a vertical fov value for the lens at the sigma web site, perhaps this is not a term thats used.
I also tried the stich in the demo version of ptgui pro and got it right at the first try. I wonder how it can now the hor fov value since it seems to be needed. From exif data? Exif data states "Focal length: 28mm" and "Focal length in 35mm film: 42mm".
Hope someone can shed a light!