Parallel tracking arms also require alignment. Azimuth, VTA/SRA, overhang.
No overhang on a linear tracker.
My Eminent Technology "protractor" is clealy more accurate than for any pivoted arm I have seen - you align the stylus to a very fine scribed line. The ET protractor aligns to the bearing housing ensuring that the stylus is at precisely the correct position for accurate tangential tracking.
For pivoted arms I use the Dennesen ( metal ) - the Feickert is easy but the size of the dot on which you place the stylus is far too big compared to the Dennesen and not anywhere near as precise as the ET. In fact a traditional accurate cardboard protractor with a pin prick for the stylus to drop into is probably more accurate than most - I have the old Garrott Bros protractors and they are very very accurate.