I've also seen guys get a tape measure refill and glue it to a strip of aluminum with the overall length wider than the wheels (around 70 inches I think).
Lift the front tires off the ground.
They then have a pointer attached to a heavy weight which sits on the ground with the pointer touching the front of the tire (center of the tread).
You then spin the tire and spray white paint at the center of the tireand you will see your line around the entire tire.
They lowered it, pushed the suspension, rolled it back and forth etc.
Took their measurement in the front, measurement in the rear and adjusted accoordingly.
They used 1/8 open at the rear
Be sure to roll the vehicle back and forth after adjusting the tie rods so the tires sit "normal". Something you can't really do with the twine/rope etc. wrapped around all four tires.
Then again, it probably doesn't need to be that acurate??

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