Set up your dado stack on your table saw. Cut a sample dado into some solid wood (same wood you'll be using for the box joints).  Measure the width of the kerf with some calipers (metric), and then plug that number into the “Kerf_Width” parameter. Enter your other parameters, and then print the Box Joint Jig.  Line up the plate (with the finger joint tab) along the kerf line of your table saw sled and glue/double-side tape it down. The other plate gets attached to the other side (doesn't need to be as accurately placed)

If your box joints are too tight, reposition the plate closer to the kerf on your sled.  If too loose, position it farther away from the kerf.

Here's a video showing it in action…