Experimental test for correct lens position within a dome-port

You don’t need to take a split shot

Distortion shows only in the water part

you can shoot any rectangular object like a grid on a slate topside and underwater and check after lens correction the distortion is the same

fisheye are easy you use the longest extension on a real dome that doesn’t vignette

the lens is distorted the technique

of two layers still works 

so in short there is no need of taking a split however a split gives you the answer in a single shot