This is a post I made on The Woodwhisperer Community forum about how to butt-joint a member into the middle of another with a Domino joiner. Normally you use registration surfaces to create Domino'd joints, but here you have a line. Voilà the post:
When you have the Domino standing up on a board, the tenon's centerline will be 10mm up from the bottom edge of the Domino. Note that this is to the centerline so it does not matter which size Domino tenon you are using (4, 5, 6, 8, or 10).
Here's a walk-through:
Voilà, two boards to butt joint:
Next, set the Domino on that line and line up the mark on the bottom of the unit to the tenon's position.