I have owned one of those very nice, very expensive aluminum alignment tools for the past 25-30 years. I can't imagine how many cartridges I have aligned... well almost aligned. After reading the good comments I purchased the Mint Tractor from Yip with thoughts of checking my already great alignment (He says as a joke).
The Mint arrived and I could tell from the directions that it had the ability to achieve great accuracy. I checked my existing cantilever angle and immediately could tell it was off. So time to start from the beginning.
In setting the overhang, my old setting was off by about .075" or around 2mm... Wow! Got it corrected and then set the cantilever angle, on the money! You have to go back and keep checking the overhang, but the more you work at it the better the accuracy becomes.
The ease of doing the alignment is directly related to the quality of the magnifier that you use. I bought both that were offered with the Mint, but had an 6x Peak (Lithco) magnifier that has a 1.75" diameter piece of glass (use the eyepiece only). The wide field of the Peak makes a huge difference and does not cause nearly as much eye fatigue.
Quality is always in the listening, and it was like adding a major component. The definition of each instrument was smooth and defined and the soundstage had depth and separation.
Obviously, designing an alignment device that is made for your specific tonearm is the way to go. In fact, I'll probably want to go back in a week or two, check it again to see if I can squeeze any more performance out of it. The tractor worked great, Yip was a pleasure to deal with and the entire Mint experience "Left a Good Taste in My Mouth"!