Unless you are dealing with those arms that don't have a slotted headshell, you usually have enough adjustment range so that you can attain tangency at the two null points, hence, you can get the correct geometry. So, even if the arm board is not drilled absolutely right, you can compensate by moving the cartridge in the headshell to slightly alter the pivot to stylus length.
If you can achieve tangency at the two null points on your protractor, you have achieved the geometry for which that protractor is designed. The problem with most two point protractors that have grid lines around the two null points has to do with ease of use. One has to rotate the platter each time one moves from one point to the other and find the precise rotation to get the needle to sit on the alignment point. Even being slightly off is NOT good. Most of these types of protractors don't seem to precisely define this point, and don't offer the mirrored surface that allows for precise alignment of the cantilever and your eye's sight line to do the job properly.
The Mint and Wally protractors not only make the alignment of the cantilever and your sight line easy to achieve, it also allows one to simply pivot the arm back and forth between the two null points without having to move the platter. That is because the null points are placed along an arc corresponding to the precise pivot to stylus distance specified for your arm and the precise distance between the pivot and the spindle.
The requirement for using this type of protractor is that the spindle to pivot distance must be accurately set because the arc inscribed in the protractor is for one precise spindle to pivot dimension. If this is achieved, once the cartridge is adjusted in the headshell for the correct overhang, the stylus will precisely follow the arc inscribed on the protractor. The null points then allow one to twist the cartridge for proper tangency at the null points. If the spindle to pivot distance is off, one may not precisely follow the arc. However, one can still achieve correct geometry by using the null points just as one would with the conventional two point protractors. You may have to slightly move the platter when moving between null points and you may also have a slightly harder time "guessing" whether you have to move the cartridge slightly forward or backwards to achieve proper overhang, but you still can get proper tangency at the two null points (just with MUCH less convenience).
The convenience factor of the Mint/Wally type protractors (or the more universal Feikert), should not be discounted. It is hard to achieve precise alignment. The more randon trial and error approach of conventional two point protractors makes it easy to succumb to a "close enough" judgment.
If you can achieve tangency at the two null points on your protractor, you have achieved the geometry for which that protractor is designed. The problem with most two point protractors that have grid lines around the two null points has to do with ease of use. One has to rotate the platter each time one moves from one point to the other and find the precise rotation to get the needle to sit on the alignment point. Even being slightly off is NOT good. Most of these types of protractors don't seem to precisely define this point, and don't offer the mirrored surface that allows for precise alignment of the cantilever and your eye's sight line to do the job properly.
The Mint and Wally protractors not only make the alignment of the cantilever and your sight line easy to achieve, it also allows one to simply pivot the arm back and forth between the two null points without having to move the platter. That is because the null points are placed along an arc corresponding to the precise pivot to stylus distance specified for your arm and the precise distance between the pivot and the spindle.
The requirement for using this type of protractor is that the spindle to pivot distance must be accurately set because the arc inscribed in the protractor is for one precise spindle to pivot dimension. If this is achieved, once the cartridge is adjusted in the headshell for the correct overhang, the stylus will precisely follow the arc inscribed on the protractor. The null points then allow one to twist the cartridge for proper tangency at the null points. If the spindle to pivot distance is off, one may not precisely follow the arc. However, one can still achieve correct geometry by using the null points just as one would with the conventional two point protractors. You may have to slightly move the platter when moving between null points and you may also have a slightly harder time "guessing" whether you have to move the cartridge slightly forward or backwards to achieve proper overhang, but you still can get proper tangency at the two null points (just with MUCH less convenience).
The convenience factor of the Mint/Wally type protractors (or the more universal Feikert), should not be discounted. It is hard to achieve precise alignment. The more randon trial and error approach of conventional two point protractors makes it easy to succumb to a "close enough" judgment.