Sony A7rii Offsets

I’m using an M2, Airpixel, and Sony A7rii for photogrammetry. I need to input the offsets between the A7rii sensor and gps antenna. Anyone know where I should measure exactly on the camera?

So the answer is, you’ll look at the camera and find this icon, the Focal Plane Indicator on the top of the camera and then measure the mm distance equal to the lens, i.e., 18mm and that is the focal length from the camera.