I measured from the highest point on the front-side and rear-side of the front tires as I could get while maintaining a tight, straight tape measure. Of course I used the same grooves in the tires, both front and back. As you will need an assistant to hold the other end of the tape against the tire, make sure your helper understands the idea of using the same groove on both sides of the tire.
After reading the geometry post above, I started wondering what the definition of toe-in really was...so I went to Wikipedia and read this
http://en.wikipedia.org/wiki/Toe_(automotive)
You know this toe-in setting is not that big a deal to do after tackling the job a couple times. Altering the setting a little bit, 1/8th inch, or so can be felt pretty quickly during normal and spirited driving. The tell-tale feathering of overdone toe angle takes more time to show so use good measuring technique and check your work a couple times, then everything will be all right.
Brian