An arc protractor is the most accurate type of protractor in my opinion. I would recommend using Conrad Hoffman's free arc protractor template generator. What is the geometry of your protractor in terms of alignment null-points? What is the mounting distance from the spindle to the center of your tonearm slot? You don't mention what protractor you are using. If you measure the distance from the spindle hole to the center of the slot on your modified template, it should be 294-mm, assuming the parameters stated in the Vinyl Engine's tonearm database are accurate.

Certainly, there is some tolerance due to the slotted design of the SME base, but your arm pillar should never be all the way to either extreme. RE: SME 3012 II mounting question, posted on Octoat 00:50:48 RE: SME 3012 II mounting question - George S. Is there any problem with relocating the arm board slot farther from the spindle even though that will put it beyond the location specified by the paper template supplied by SME for this arm? This is unsightly, and I feel a need to adjust the location of this slot rearward on another test arm board. In fact, the tonearm base is overlapping the grommets on the mounting plate. During alignment of the cartridge, using a protractor supplied by my cartridge mfg, the tonearm mounting position ended up at the extreme rear of the arm board slot. I used the SME template to locate the slot in the arm board and cut it out as accurately as I could. I am prototyping the mounting of an SME 3012 Mk II tonearm on a Thorens TD-124 Mk I turntable.

Sme tonearm length