The one I don't get, so hopefully someone can explain this to me. How can you be sure that the curvature of the wheel isn't affecting it. I know that the toe is the difference between the two, but it seems like the rim or tire could alter the results.
If your toe plates are in length, the same as your tire diameter, and your tires are inflated identically and you position the toe plates the same onto each wheel/tire, your measurements will be accurate. I've watched some alignment guys use the "chalk and string" method, some using a solid rod with adjustable scribes, and they just don't appear to me to be as accurate as the DIY toe plates. I have been using mine for about two years now and they are "spot on" consistent with an Allen Lemons alignment (which I get every year just to check everything out and make sure I'm not missing something). I used them extensively on Jeff's Subie last year and while the Subie's suspension had some settling issues, the toe plates always gave me a reliable reading. I swear by 'em and recommend them to anyone wanting to adjust toe settings at the event.