OK, so I popped for the Really Right Stuff pano head PG-02LLR. What a beast - I didn't get the vertical arm w/ cradle style clamp. When I am shooting movement, I can get to the center of gravity, or at least really close. Not sure I need the vertical arm & cradle, but we shall see.

From my understanding/perspective the side mount aligns the vertical center of the lens by default.

To find the offset on the horizontal base I used extension tubes w/ the camera/lens pointed straight down and lined it up w/ the crosshairs - Magic Lantern has a feature to find the exact center of the sensor with a uber small box for the magnification window. I used it to line up the with the markings for the base for center of rotation. I did this for my camera body (L plate) and my 70-200 (lens foot). The camera body setting should be the same regardless of the smaller lens mounted. The offset are 89 (vertical orientation) and 96 millimeters respectively. I won't be able to do this for my 150-600 but I should be able to really close by measuring the diameter of the lens ring & foot compared to the 70-200.

Now I need to find the nodal point, which is a trial and error effort for Saturday morning.

I am impressed w/ the fit and finish, I can see the difference of a Pano-Gimbal head vs. a Gimbal head that is focused on the super/uber telephotos.

I am good w/ the 150-600 on this, a 600 f4 etc would not break it, but I would think a dedicated Gimbal would be more functional for a swinging set of bird shots, etc. with such a hefty lens.