An alignment protractor is used to find the correct distance from stylus tip to tonearm pivot. An alignment protractor is a plastic template onto which are printed the null point(s) and lines of tangency against which the cartridge should be aligned.
The template is placed over the turntable's spindle (made possible via a spindle-sized hole drilled in the template) and placed against the platter.
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.