Gunso said:
I don't quite understand 'reference surfaces' on 90 degree joins yet.
When you hold (or clamp together) the two pieces you want to join together in the intended configuration:
The two surfaces where they are touching are the ones where the mortices will have to be made, these are the reference surfaces (where the business end of the domino, where the drill will push out, will reference against).
Pick two edges (one on each piece) parallel to each other (preferably directly and flush touching each other), these are your reference edges (where the corner between the business end and the fence, set to 90°, of the domino will register against).
The other reference surface will be the one where the fence will register against.
Draw a line across the two fence reference faces, perpendicular to the reference edge, to mark the position of the mortice.
Separate the two pieces, reference the domino to your reference surfaces (leading to it being snug against the reference edge), align the sight gauge center line (see
https://service.festoolusa.com/media/pdf/Domino_DF_500.pdf page 22) to the line you drew (keeping the fence and business end both flat against the reference surfaces), hold the domino in place with the fence handle and then do the plunge, slowly. Repeat with the other piece, dry fit a domino to check if all is well.