The reason for 6" off the floor, is that most of the wheels are out of Round, even when they were New. To check this on your car, jack up the front end, in the center of your axle with a Floor jack. Spin each wheel and watch the Tire Thread and Wheel Wobble.
When I set the Toe-in, I raise both wheels off the Floor, Spin the wheel and using Gray Primer spray the center of tire about 1 inch wide pattern. Then take a Vise and using a #8 penny nail in the vise at a up angle, spin the wheel to the rear to mark a slight groove or mark into the Paint.
At 6" off the floor front and back, shouldn't be that far out of round. Going any higher, you would be defiantly be in a out of round area, as you saw when you spun the tire. Really shows up out of round when you spin tire with mark in the paint.
Take out the jack, both wheels now on the floor, measure up 6" front and rear of both tires, make a Horizontal line. Another reason for 6", is the ease to obtain your measurement. Using a helper and a Tape Rule, or a 1' Square stick, mark the distance between both lines, or get the Measurement. Using a Pipe wrench, turn the Tie Rod until you get + or - 1/16 distance between groove in paint. Do Not Roll the car back or forth, It must be held perfectly still as to not change the distance between scribe lines. Works for me, and for the heck of it, one time I had the car at a Alignment Shop and Had them check it, I was really close.