Hello Nrenter,
I've had a few Protractors over the years, first the Mo-Fi Geodisc about 10 years ago.
Then Doug Deacon suggested the Turntable Basics Protractor to me, and I thought it was better than the Geodisc, being a Mirror, having two Null Grids. Doug explained-taught me how to enhance its aiming acuracy, with the use of a a piece of thread, to extend the Sight Line to Tonearm Pivot. I felt the sound did indeed improve.
I think maybe the T-Basics Tractor's one flaw is thicker Grid Lines, and one doesn't really realize-know this until something better comes along.
Print-ups from Vinyl engine just didn't seem worth the effort, they were in no way better than the Geodisc, or TB Tractor for Baerwald Alignment IMO.
Then for a period, Ken Willis, and I had collaborated, and Ken kindly emailed some PDF files of some Arc Tractors for my particular S-P Distance. Here again, I noted errors, and a slight improvement, versus the TB Protractor. Things kept getting better, and sounding better, so at least I knew I was headed in the right direction.
Then enter the MintLP Protractor. I like others, took the chance, there was lots of very good buzz about this product, giving good gains in playback performance, and like others here, I wanted to reap those benefits as well.
Of course one might find it hard to shell out $100-$110 for a tool, and be aprehensive-skeptical of its worth, I was a bit at first I admit.
The gains I feel was worth the cost to me. Before, my sound seemed to sound "almost right, not bad, getting better, but was still feeling like something still wasn't just right, or correct". After the careful use of the Mintlp, the sound was improved to the point, that I feel this is now one area, where I haven't induced any incorrect set-up errors, from lack of knowledge, or expertise-experience.
I'm certainly not anywhere near the league of Frank Shroder, who can listen to a Table-Arm Cartridge, hear certain shortcomings in an instant, and quickly, and wisely know "the VTF is too high, VTA is off, there is inner groove distortion being caused by this or that, etc." (in other words, to make a long story short, an average Shmoe like me needs all the help I can get!) This one tool has helped me to better my sound, and to better understand this one facet, the geometrical mechanics of achieving optimum vinyl playback.
Ken Willis, like you, has made very recent mention as well, of perhaps manufacturing a Mirror Arc Protractor. I wish him well in his pursuits, he's a great guy, and helping to provide good tools, and help to the audio community.
As I close, one very unique property of the Mintlp Tractor, is that the Vinyl Overlay on the Glass Mirror is a translucent material, except where the two Null Grids Are. There, the material is trasparent-crystal clear in that area, thus aiding in better ease of accurately sighting of both the Null Points, and the Alignment of Cantilever. This man apparently had his thinking cap on when he made this Tool. Mark