There's a slight caveat to the 1/3 rule relating to comparative strengths of the workpiece and the Domino tenon, and the width of the joint in comparison to the width of the Domino tenon.
Similar strengths = 1/3
Wide joint / narrow tenon = 1/3 or greater
Softer woods = 1/3 or less
Plywoods / MDF = less than a 1/3
There's another advantage too with sheet goods, especially given some are undersized. If you used a 6mm Domino tenon with 18mm or 3/4" plywood, it's 40mm long which means you have to cut 15mm in one part and 25mm in the other and *remember* to change the depth before you plunge!
In the same 18mm or 3/4" plywood, a 5mm Domino tenon is only 30mm long and so 15mm in both parts without the risk of blow out because you forgot to change the settings. And it helps to maximise the strength of the substrate in the joint. You also get 300 5mm tenons for a similar price to 190 6mm tenons...
