There is a simple but very good diagram shown on the "Barrus" Mariner website.
Engine Choice | Barrus
If you copy and paste the images so that the outboard engine is adjacent to the transom you will see how the anti-ventilation plate lines up with the bottom of the transom and advice on the mounting position at the transom head. I have a Mariner 20hp and the instruction manual recommends the anti-ventilation plate is inline with the transom bottom with a maximum tolerance of 2 inches below. I don't seem to be able to paste the image on this site but can e-mail it to any forum member on request. Hope it helps?