I did a third Synth, this time I got it 100% synthy. In this try, I used about 30 images of Karlsruhe, I did during a night session. I also used a panorama, I created out of a few of this images. This seems to be a quite good approach to help Photosynth to create the good Synth.

I've also used the first time the map feature you can find at your Synth's page.

You simply select the place, where your Synth belongs to and save it along with it.