Dewalt Fixed Base Routers come with a clear, LEXAN sub-base which offers optimum bit visibility and base durability. In order to center the sub-base a sub-base concentricity gauge is included. If you insert this into the collet prior to bit installation, it will help you center the sub-base relative to bit location when making your cut line off of the side of the base. The video below demonstrates how this works.
Router Advice And Tips, Routing Videos