I use 1/8″-deep dadoes for this joint. That fully serves the purpose of the dado. In prototypes, 1/16″ was too little for comfort but I suppose 3/32″ would be OK.

The sliding dovetail is a great joint too. I have found that a non-tapered sliding dovetail longer than even about 2 inches or so is a royal PITA. So, use a tapered sliding dovetail for > 2″. The depth of the dovetail should be about half the thickness of the board but probably never more than that. For 3/4″ stock, I’d say the minimum depth for a decent dovetail lock is about 1/4″.

Keep in mind that the cross-rail joint is a poor choice for joining a shelf to the side of a bookcase because the half of the joint in the side of the case will not have long grain-to-long grain glue surface. The cross-rail joint would be good for joining a shelf to each of the opposite sides of a vertical partition in the case. For joining a shelf to the case side, go with the tapered sliding dovetail, except at the very top and bottom, of course.

Hope this helps. Good luck with the project.

Rob