I have one myself. Had to remount it twice. It needs to be away from the engine. 15 inches sound about right. "Keep it away from the bottom of the boat!" About half way between the bottom of the boat and the waterline on your transom would be fine. Some fiberglass boats have different shaped transoms, but the transducer bracket should be adjustable so you can mount it to a curved transom, but still be able to adjust the tranducer to a level position.

Ray