Mercury Mariner Outboard 225 3 Litre Service Manual 1994, Page 326Get this manual
INSTALLING OUTBOARD ON TRANSOM Locate (and mark with pencil) vertical centerline (a) of boat transom A
inches (660mm) centerline to centerline width is recommendedThis is required to alleviate cowling interference during lock to lock turns if one outboard would be in the full tilt position, while the other outboard(s) are in the vertical running position1Secure (tape) "Outboard Mounting Template" (located inOutboard Installation Manual ") to the outside of the boat transomedge of (real) boat transom, not shimsB
2Align "Top of Boat Transom Une" on template with top