Patrick (GulfcoastGuy) is exactly right! Ive been using a jig like this for many years - and have never had a problem cutting dado slots perfect every time! I will say this, though; although it is unavoidable - I despise using fasteners into the end grain of plywood. There is a work-around which is to predrill a small diameter hole for the fastener (through the ungrooved side and into the end grain), then drill another larger-diameter perpendicular hole downward for a wooden dowel. Carefully planned, this enables the fastener to go into long grain of the embedded dowel - screws are preferred over nails. Trim the protruded ends of the dowel after the fastener is fully seated.
OPG3 Tweak everything!