"Toe-alignment with a string only works if front and rear wheel-track is exactly the same and equally sized wheels and tires are used.

So measure track-width front and rear seperatly. If they differ, don't use a string because you'll end up with a tire-scrubbing toe-in or toe-out."

Not sure where you are getting this information, 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.

It is critical to place the stings 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.