You also need to measure the interior offset. I did not want the gasket compressing under the square, and throwing my reading off, so I removed the gasket/seal on the Beckson ports to get a more accurate reading.
Note the gasket grove or slot on the port and the staining & grime from leaking. I had totally cleaned each of these slots or groves last spring and installed new gaskets, yet a few of these ports continued to leak. I guess plastic, in the marine environment, has a useful lifespan that had been surpassed..