1/2 way up the tire front and rear. Center of tread to center of tread on the other side of the tractor.
However, the tractor engine etc. gets in the way so you need to be as high up as you can get using the tape mesure under the machine. Like I said, side to side. 1/4 inch wider at the rear will give you "Toe In". If you get 1/4 inch wider in the front you will get "Toe Out".