Read Richard Babbit's article on the topic

Most woodworkers tend to dial in machinery settings like depth of cut by making repeated test cuts on small scraps. While this is an excellent method, you might also consider adding a height gauge to your woodworking arsenal.

These super-accurate gauges can be used in a variety of ways—from setting tablesaws and router tables for depth of cut, to checking the alignment of a jointer's cutterhead. In this short video, Fine Woodworking's managing editor, Mark Schofield, shows you how to properly use a height gauge, and then takes you through a variety of workshop scenarios where these handy tools can really shine.

More on Machine Setup
Low-Tech Tablesaw Tune-Up
Setup Kit for Machines
Tune Up Your Tablesaw
Use Test Cuts for Accurate Machine Setups

Of course, height gauges measure in thousandths-of-an-inch, not the best system considering the fact that most of layout joinery in fractions. So be sure to download the free measurement conversion chart.