I'm sure others will have additional input, but I follow the same guidelines that I would use to determine the tenon size when making a mortise and tenon joint. For example, when using 3/4" stock I would usually make the tenon 1/4" thick so that the amount of wood left on either side of the mortise is a 1/4" also; so about a third of the wood thickness in that case. There are exceptions, of course. The only important thing for me is the amount of wood remaining on either side of the Domino slot across the thickness of the wood. For me, sufficient support on either side of the mortise translates into ensuring the integrity of the structure.