I have a Domino Setting Gauge. It's on one of my DVDs but I forget which. One of the early ones.
It consists of a piece of Perspex with a line scribed along one edge. The distance is exactly the distance from the base of the Domino to the centre of the cutter. 11mm if memory serves me right, which is does very rarely these days.
In use, the scribed line is placed on the centre line of the cut, a wooden batten is clamped up to it to make a reference fence and then, when the base of the Domino is located against the batten, the slot will be exactly on that original centre line.
In my case I have done one edge for the Domino and the opposite one for my BJ. It's a jolly useful gadget for situations where the mortice is further from the edge than the machine's own fence will allow.