One thing I do stick with is a single width reference domino to keep the boards aligned, then double width elsewhere. Referencing from the base the 6mm should be fine, tolerances are pretty tight in 18mm and it is better if you set it up for dead centre off the drop down fence but I've forgotten how to do that!
If I find my notes I'll post them.
Ply is tough on the cutters, I fitted a double decker bus with ply furniture and wore out a cutter, only realised when I was having to hammer the dominoes in place.
Had a look at mine today putting it back in the box and it was made in 2006! By now it's probably saved me the purchase price in screws.