Sometimes you need to drill evenly spaced holes. If you have a ruler at least half as long as your workpiece, you can do it accurately without a lick of math. Here's how.
Drill Evenly Spaced

First, measure, mark, and bore the standard's center hole with your drill press. (If you need an even number of holes, locate and bore one of the two holes closest to the center.) Keeping the bit in the hole and the shelf standard against the fence, clamp a ruler to the fence so that the end of the standard lines up with an easy-to-remember increment (for example, the 11" mark as shown at top.)

Now, raise the drill bit, and move your workpiece so the end lines up with the next spaced increment (for 1" spacing, slide the workpiece to the 10" mark; for 12 " spacing, shift it to 1012 "). Then, bore the next hole. Continue to the next increment, drill again, and so on, until you reach the end of the workpiece. Turn the standard end for end and repeat the process, starting again with the center hole.
Chuck Hedlund, WOOD® magazine staff