Scott,
The front axle centerline is 20.594" inches back from the front edge the frame. Place a straight edge like a 3' or longer level against both front framehorns and measure straight back, not along the frame rail as they taper to the front.
The factory assembly manual has detailed frame drawings, and would be a good thing for you to have. And it's always good to have more than one reference point like the one given in the previous post.
