Anyone who uses stacking dado cutters soon discovers that “x” inch stock may not exactly match your “x” inch dado setup. Instead of trying to adjust the dado cutter with shims, I’ve found that it’s a lot simpler to make a series of trial cuts in scrap hardwood using standard combinations of cutters – 5/16″, 3/8″, 7/16″, etc. Then you can use these “gauges” to plane the stock to the exact size to fit a particular dado slot.
– Jason Landers