Cheapest, most cost efficient route for a good Protractor, would be the Turntable Basics Mirror Protractor. Better choices, but more costly, would be the Ken Willis Arc Protractor, or the MintLP Protractor from Hong Kong. This Arm uses Baerwald Null Geometry
With either Arc Protractor, one should accurately measure, and insure what the Spindle-Pivot Distance is, and not assume it is at 211mm. With the TB Protractor, one will not need to know S-P Distance, but will need to accurately aim the sight line, and this can be enhanced with a piece of thread stretched from the sight line to the center of Arm Pivot. Mark
With either Arc Protractor, one should accurately measure, and insure what the Spindle-Pivot Distance is, and not assume it is at 211mm. With the TB Protractor, one will not need to know S-P Distance, but will need to accurately aim the sight line, and this can be enhanced with a piece of thread stretched from the sight line to the center of Arm Pivot. Mark