here you can see the spacers.
They are useful so I can see exactly where the router bit will cut the dado.
I made one with a 8mm router bit and one with a 10mm router bit.
Theay are easy to make. Just screw a piece of mdf to a straight edge, route a dado on the mdf following the straight edge, unscrew, cut waste where the screws were and you have one spacer
[attachimg=2]