It's a great theoretical method but it's a lot harder to take measurements in the range of 1/32 to 1/64 that matter in toe alignment than detecting a deflection in a straight string which you can easily tell when the string goes from touching to bending. It's quite easy to get within 0.05° angle using a string.

I was originally going to use a laser with a magnet to send a dot to the garage door and measure the distance between them left and right until I discovered the string method that road racers do to set up their toe

The problem with only referencing the other wheels/tires is that you can end up with a perfect alignment that is "crabbing" down the road. Unless something is broken or damaged from an accident, I doubt that will happen. Also it will keep the tires from destruction even if the car goes down the road at a slight angle from straight.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)