By definition it is the height the factory puts trucks at just to help companies like Belltech, DJM, Air Ride Tech etc. sell lots of parts to the buyers of these stock trucks so that they can lower them and thereby make them look cool ;o) Sorry to be such a smarta$$ Sometimes I just can't help myself!
On a more serious note if you tell me where you want it measured from, like say from the center of the wheel to the top of the wheel well opening I can measure mine then add the amount I know I lowered it to get in the ballpark. It should be a fairly accurate number? I'm guessing that you are talking about a 2wd right?