The Precision Router Depth Gauge designed by Jim Stegall (of '70's F1 Stegall Minnow fame) is now available on Amazon at a incredible 40% discount for the Holiday season at
http://amzn.to/2AVl5H6 ... Only 4 complete units available, but around 50 bases ready in stock with more gauges and the "foot" for the gauge on the way.
If you use a router table or hand router for wood working, this is the easiest and most precise way to set your bit depth. You can easily change bits and get back to exact matches. Just make sure you write down your measurements
Even though this product is primarily for hardwood woodworking it works well for small handheld routers that can be useful building model airplanes, or plugs for molded airplane parts. I use one to set a Dremel router attachment for routing aileron slots in sheeted foam cores for Quickie 500 planes.