Just adjust the router depth and test it on some scrap until you get the depth just the way you want it.
Yes, test on the scrap, but you can get the depth you want the first time .
This assumes the use of a plunge router, as I've never used a fixed base one.
Set the material you want to route out for on a flat surface. Set the base of the router on the material, with the blade off the edge slightly.
Lower the blade until it's touching the flat surface, and lock the router down. Viola! Instantly set!
Not my idea, but Oscar's.