Think that Dawson does not use it is that you have to take the sight off the pistol in order to measure it. If you have calipers you can get the measurement from the top of the slide pretty easily. I change both sights so the measurements on mine would not do you any good.
If you have the measurement on the sight and know the sight radius and how far you need to move your group, you can call Dawson and they will assist you with which one you need to order. They have an application on their web-site, but I can never get it to work.
Wish you the best of luck.
JW