1/4" bit setting for Porter Cable 4215 jig

I wanted to do half blind dovetails but the depth settings for the bit (1/4" bit) don't seem even close. I'm using 1/2" boards and the pin side is too big for the dovetail. I tried setting the bit shallower but that ends up not really making a dovetail. Unfortunately setting the bit deeper ended up breaking the bit.
When the new bit arrives I'd like to have some method of calculating how far the bit should extend from the router.