Originally Posted by ichso

A new attempt 2 days later:
Lat: 47.5°N
So referring to the Van's Sun Almanac and interpolating between 40°N and 50°N again I should have seen the sun rise at 7:49 ( 8:49 loacl time at 15°W)
I tried to use the 'red light off' moment as my local SR time. this was at 9:10.

So I am 21 minutes behind the SR time for 15°W which would mean that I am ~5° farther west, so at ~20°W, which would meet my real longitude well (still being ~20.5°W).
I am not sure what to take as my referring point for the SR yet but the moment when red light turns off (= sun is to be seen by half of it's amount) fits well in this example. I will test this a little further tomorrow.