Hmmmm.... maybe I am not understanding all this, but I do believe that 4x gets me only to the front of the lens (that is where the base of the dome is - exactly at the lens front). Unless I'm missing something, I also have to add in the distance from the front of the lens to the sensor. If I add all that, I'm at ~ 10".
I'm not saying you're wrong. It's just what I've observed (and tried to learn and absorb) so far.
Again, I'll take time to do some more accurate and in-depth testing in the pool. I can lawn lay down a tape measure, and take more detailed tests.