PV Sol Expert does the row spacing calculation for you if you specify the dimensions of the frame and select the appropriate panels. From what I could tell their calculations are based on there being no shading on the rear array at 12 noon on the winter solstice (if your array is facing due south). However, If you model this in other software or look at the animation in PV Sol you will notice that either side of 12 noon you will still experience some shading on the lower corners of the rear array as the sun dips towards the horizon.
If you want to be able to visualise this without spending a grand on the software then download Google sketchup, it's free but will take a bit of getting used to. I also think that the images look much more professional that the very basic graphics produced by PV Sol. In Sktchup you can grab your actual location and it displays an accurate angle of the sun's path for a given time of day if you activate shadows. Example is shown below:
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The image above shows the angle of the sun at midday on the 21st December for a recent project we completed. I wasn't happy with going with the distances specified by PV Sol due to the shading either side of midday and there was a bit of space to play with so we moved the rear array back a reasonable amount. The diagonal line from the near top corner of the front array is the angle of the sun at noon - the minimum spacing. In fairness I don't know if this will have a massive impact on total output as this is only really an issue during a poor month for generation.
Image below shows same installation again but at 8:50am on 21st and highlights the most significant shading situation, still pretty minimal.
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Hope these images enlarge OK.