Unfortunately, using the framing square will use the running bottom as the datum, which might or might not give you the correct angle, i.e., if the bottom is on a 3-degree downward angle running to the stern, you just included those 3-degrees into the included angle for your resultant value.
I recall when I was working bracket companies for that Stratos we added a full transom and OB bracket onto, that they needed the transom angle not to include any influence from the hull bottom. Stratos told us the proper pitch to rig the boat up on the trailer (to achieve the correct hull attititude) and we used a plumb bob and protractor.
For a Parker, I'd just call the factory ...