Dont know, but heres how to cross check your method.
Open wheel car...pretty hard to beat two jackstands, string, and a tape measure. That method also lets you check rear axle thrust angle, though its nice to do that with another pair of jackstands and more string. You should set the front wheels to zero while checkng the thrust angle this way, just to be sure you are dead straight to your Ackerman and scrub radius.
You're just running the string loop across the tire faces (wrapped around the back of the tire, tied off to the axle) and moving your jackstands in or out to get each side parallel. With the jackstands 6-10 feet (or a lot more if you have room) in front of the tires, you get very fine accuracy. You are measuring the strings, out front, to get them parallel, and then your toe-in shows as a small gap at the forward sidewall of the tire. Comparison check the tire sidewalls to the rims, to be sure you arent measuring a bulge.
With the front tires dead straight and parallel (no toe-in) you can pull string from the rears, passing the fronts, to measure for parallel and thrust angle. If the rear tires are narrower than the fronts, use wood blocks vertical to space out from the sidewalls.
Note about the wood block trick....pretty handy for using this method on fendered cars. Just make an H out of 2x4s that can stand against the sidewalls and pull your string loop. Dont fuss with string tension by retying....just slide a jackstand forward.
When you finish, do the water puddle trick on each side to make sure you are right (or that you werent actually straight...I've caught myself on that one). On flat concrete or pavement, pour a water puddle on the ground and hand roll thru it with the wheels dead straight. You can measure any offset right there on the floor when the front and rear tracks show on top of each other. (This old trick can be pretty discouraging when you just bought that brand new, whiz bang, super duty dually for a gazillion dollars. Just take a jug of water to the Walmart parking lot on a Sunday morning, drive through the puddle straight, cross your fingers and then walk back for a look-see.)
No matter how you measure or what tools you use, failing the water puddle test is the final answer for thrust angle. Flat ground and tires dont lie.
Maybe this is a dumb post and I am preaching to the choir, but sometimes we forget how well the old ways still work. A fellow starts getting a little gun shy when the thinking starts muddling with age, so I have to delete most what I write these days.
JimL
