Ok... I have a centering ruler that's 18" and use it all the time. It's steel but I don't really need the see through thing or special markings, as I know the measurements I use. I like the 18" length as I use an odd riser blank pattern I call an "overpass", that serves for either my forward riser deep locator model or a standard longbow riser. My risers can run from 16" to 18" and I use the center finder to set the exact center of the riser for either model, or to slide down to set the 4 1/2" top flat surface I use if I'm going to grind the overpass off for a standard riser. Neat tool. They're commonly available at upholstery supply shops, and probably on the internet if you google for "center finding ruler". I got mine from my wife when she bought a new one for her business.