You will see that the VPI is quite enough. I also have the DB protractor, and after checking with Harry, he said that I could use that one too. It was exactly the same with them both.
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Viper, if you think about it or have seen diagrams of cartridge alignment, it is possible to get it correct with a single point, rather than two. I'm guessing the two point systems are more common since they come from identifying where tracing of the arc with a pivoted arm would cross the tangental line as scribed by the cutter head. To achieve minimal tracing error, the arc would begin below the line of tangency, cross over to a point just above it, then cross back to end up just below it. Several systems were developed with different objectives for minimal tracing error such as overall arc, inner groves, etc. (Baerwald, Loftgren, Stevenson, etc.) where this arc crosses the line of tangency. Much has been written about this if you care to do a search.
Even though the arc traced by the stylus crosses the line of tangency at two point, a third point between them but along the arc can be identified to simplify the alignment process. This is the basis for universal protractors such as the Feickert and Dennesen as well as arm-specific alignment devices such as VPI and Graham.
Either type, carefully used, should provide accurate alignment. Sonic variations should only come from the differences in system objectives.