I can't use mine either because you can not rotate it to 0 degrees if you use a 20 MOA Picatinny rail. A design flaw.
20 MOA is an entirely different story than 20 degrees(~19.666 degrees different). I'll admit to not having had one of these in my hands. In the photos it looks to have 2 degree hash marks and an indicator line at least as wide so accuracy to one third of a degree is impressive!