[Fr. Frank]

There's a formula I can't remember, wherein you take the long side of a carpenter's square and put it under the keel line with the hull and up against the vertical chine, with the short arm of the square pointing up. You then slide the square as far under the hull as it will go, until the end of the short arm of the square makes contact with the transom. Using the height above the hull bottom for the contact in relations to the distance of the inside corner of the square from the transom will give you the transom angle.

An online transom angle calculator can be found here: http://www.rocketflap.com/transom_calc.php