Before gluing up the main cabinet, I needed to cut the dado joints for the shelf. I wanted my shelf to sit 3" above the bottom and a 1/2" from the front. Don't forget that the back panel is offset by 3/4" and is just shy of 1/2" thick, leaving about 5 3/4" between the back panel and the front of the cabinet. Factoring the 1/2" inset from the front, we get a 5 1/4" shelf. There are always subtle variations during the course of a build, which changes the actual measurments of the various components.
I used the Festool OF1400 with the 1/2" straight bit, MFT/3 and Guide Rail to make my dados. I realize that not everyone has this setup. A router, straight bit and any straight edge will work just fine.