The rule of thumb is 1/3 of the material thickness.
For 1/2"take 4 mm, for 3/4" take 5 or 6 mm.
Normally Dominos are placed in the middle of the material, but placed at 10 mm from the top of a shelf is easier when doing vertical plunges, because the offset of the DF 500 is 10 mm.
And the forces from the shelf to the corpus are from top to bottom. So more material on top is better. But then you have to take care of the dimensions you are using.
By the way, most sheet materials are not exact 1/2" or 3/4" thick, there are always slight varieties in thickness.