I just draw a line across the drive and pull across it. The chalk will show on the tire and you'l know how much tread/pavement contact you have. FWIW, i use BFG A/T KO's and use 36 in the front and 34 in the back. Just remember to rotate those little doughnuts every 6K miles or so for max tread life!!

More like feet than miles.  Also more like section than line.

The way I did it was to chalk off 6" or so all the way across the tread and then drive a couple hundred feet in a straight line.  Adjust air pressure appropriately and repeat.  I always start with high psi and go down but I guess you could go the other way too.