How critical is the exact placement of the tonearm

It is amazing how good a tonearm/card will sound even if pivot-to-spindle is off by only .03 mm or so. But it is amazing how much better it does sound when you reduce that error even more.

Hitting both marks on a two point protractor makes one very huge assumption. That is, the pivot to spindle is dead nuts on. If it is not, there is nothing you can do to correct for it as the tonearm swings the arc. Sure, you might hit it at one point and it might be close a little farther into the swing but at the opposite end you're going to be off. It is simple geometry.

That's correct, Dsa. The proprietor, Yip, makes the protractor for the tonearm parameters you specify. I had given him the numbers for the protractor I need without the name of the tonearm and table. I guess he is building a database as he asked for these before accepting my order. Another thing that came out in the thread on Thom's forum is that you will also need to specify the spindle hole diameter. Yip does cut to high tolerances so keep that in mind with the spindle hole. Oh, I also like the way he personalizes them.

Yip also asked if I wanted a lupe he recommends for seeing what is going on while adjusting with his protractor. I ordered one since I liked the look of it and feel I can never have too many tools for seeing what I'm doing.

What I like about the arc-style protractor is that you have a single tool that allows you to get the pivot to spindle right at the same time you get the cartridge alignment right. It is possible to get things right with most any 2 point protractor, but you would still need a tool and method to measure the pivot to spindle to get it really, really close. So if you are getting accurate results with a 2 pointer, that's all you need. In my case, I didn't realize how off I was until I tried the arc-style. I was clearly off the arc. Once I got things adjusted closer and re-checked using my 2 pointer, I found I was off by less than .5 mm with pivot to spindle. But the difference in the sound of reproduced music was noticeable, especially in the articulation of consonances. Well, that's what I noticed first. I suppose the benefit is to all micro-dynamics. Getting this adjustment closer to theoretical perfection provides for just that much more of an "in the room" presence from practically every LP I played since. There is probably still a small error in my setup but I'd say it is limited to the width of the printed arc. Once Yip's protractor arrives I'll adjust things again.

I agree, Doug. It's not that one protractor is superior to another. All should work very well if they are used properly. But one could work better for someone versus another style protractor, depending simply on which may seem to be more user friendly. However, getting back to original question, the placement of the tonearm is still very critical to get right no matter what protractor one uses.

I guess I don't in either Swampwalker's or Doug's "eyesight camp". My eyes have gotten bad in both near and far sight over the years. Good thing there are also great optical tools available as well as great protractors! It really is an excellent point. I think it is easier for me to see of the stylus is on or off the arc. Once I have that, squaring up to a single point is simple. As long as I don't move the cartridge screws too much forward or back, things are good. But re-checking the alignment with the arc after squaring the cantilever up is still a good idea. Just as with the two point protractor, it is an iterative process.