Mike L,
The first photograph is a head-on picture of the stern of a Merrymeeting Bay Gunning Float.
The second photograph shows the sheer measured to 36 inches. Notice that on this boat, the scull port bisects the sheer line. This is different than John Gardner's boat.
The third photograph shows that the stern measures 12 inches from its top to its bottom at the keel.
The fourth photograph shows where the sheer line and freeboard line bisect at the center of the stern. Notice where the sculling port is in relation to the bisecting lines on this boat.
Also, there is a plastic inserted in the stern that houses a stopper to keep water out when the scull oar is not in use.
The fifth photograph shows the sculling port from inside the boat with the stopper removed. Notice the sculling seat in relation to the sculling port.
My hope is two fold: (a) that these pictures may help you think about placing your scull port in relationship to your boat and (b) to compare these pictures with the drawings Steve Sanford provided you above.
I have additional photographs of the measurements taken of the scull port itself in relation to the stern if you think they will help you in measuring your scull port.
Please just let me know if you would like to see those photos.
Cheers!