This video shows in detail how to make a custom division plate for the ultimate box joint jig and the advanced box joint jig. This is also a very good way to deal with metric size blades, such as 3mm kerf blades. In this example I used wood, but you can also use hard plastic or even solid aluminum. The benefit of those materials is that you can cut the plate with a finer spacing, like 1/8" (rather than the 1/4" shown in the video).
Later, I added another pointer to the left side of the tee to use the stop on the right side wing as well. I may add a scale to the telescoping stick to make that read from the main scale on wings, but strictly speaking I don't think it's needed.