When aligning a cartridge for tangency using the alignment protractor, it is essential to remember that you are attempting to align the cantilever (and, hence, the stylus), not the cartridge body. There is no guarantee that the cantilever is perfectly aligned within the cartridge body, so simply aligning the cartridge body will not necessarily produce the desired result.
Furthermore, many cartridge bodies have non-parallel sides, making tangential alignment of the cartridge body with the lines of tangency on the gauge virtually impossible.
Cartridge must be adjusted until the cantilever is parallel to the set of parallel lines. And this should be achieved for both the indicated points. When the cartridge's longitudinal axis is parallel with the horisontal lines, tracking error will be at a minimum.