Not sure where you are getting this information,
First hand info, but... most folks (like me at first I admit) will put the string from the back of the rear tire to front of the front tire, per side.
If done this way while the car's front/rear trackwidth differ, the frontwheel toe-in will be wrong.
but the strings are only parallel lines to measure from. My front tires are considerably smaller than the rears, and sit inboard about 1/2" to 3/4" from the stringline taken off the rear tires.
You'll have to find a fixed point in front of the front tires to mount the strings on, with left and right being equal.
It is critical to place the strings so they just barely touch the front edge of the rear tires. The back edge of the string wraps from behind rear tires and along the rear edge of the rear tires. Once positioned correctly, the distance from the string is measured at the front and rear side of the front tire with a tape measure. The front measurement is a bit less to allow for toe-in.
Agree with that. But I would focus on the front tires being (almost) parallel to each other, by either placing a string/tapemeasure between the front and rear of the front tires.
