My Panda has setting marks for the wiper arms etched into the 'screen. They're a pair of small dots which line up with the ends of the wiper blades.
The idea is that you cycle the wipers with the arms off, then replace the arms so that the ends of the blades line up with the dots.
Now my memory may be failing me, and I could be confusing this with another car, but IIRC the wiper shafts on mine are tapered, not splined, so the arms can be set anywhere and therefore can be adjusted precisely. However, just like an unkeyed cam sprocket, this offers endless possibilities to go wrong and get into difficulties, rather than just being a spline (or tooth) out.