Pro tip: I used to cut dadoes with a router jig and a prayer, now I stack two blades and check with a $12 caliper
For about 3 years I cut every shelf dado with a router and a straightedge clamped down, and I'd still end up with a couple that were a hair loose and needed shims. Then a guy at a shop in Fresno showed me how to stack two dado blades and dial in the width with a caliper, and my dry fits got tight enough that I barely need glue to hold a shelf. Anyone else run a stacked dado for cabinet work, or do you stick with the router and just deal with the cleanup?
Honestly a caliper doesn't tell you the actual cut width, it only measures your blade stack. The real test is a test cut on scrap, since the arbor and flange slop can throw things off by a few thousandths.