" I measured with a string from the rear edge of the rear rim to the front edge of the front rim"
If you accurately described what you did, then I would say you got it all wrong.
You only measure and deal with the front rims. As best you can measure from as far forward as possible on inside of one front tire to the same place on the other front tire. Then do the same at the back end on the front tires. The front measurement is easy and you can do it half way up the tire. The back is tougher because the body of the quad gets in the way of measuring half way up. Just get that measurement as high as you can.
The easiest way to make a small adjustment is to loosen just one end of the tie-rod end. I removed the entire ball joint end and turned each one, one revolution "out" lenghtening the arms and decreasing the toe out. Just be sure to do them both the same unless there is an obvious difference side to side. You can measure nut-to-nut on the rods to insure that they are starting and ending up the same.
Try it one turn each side if that doesn't suit you then try another; but stop if they start to point straight ahead or inward. The only thing critical is keep both side the same; get the feel and handling the way YOU like!
Mine is out at the front about 1/4" to 5/16" and handles more to my liking than the original 3/4" out. There is mush less wander now.