Nauticam port and extension ring

What is the proper extension ring size for a Zen 230 dome being used on a Nauticam Mark 3 housing, using a 17-40 lens? The nauticam port chart does not list Zen domes.

On the Nikon side the 16-35mm works nicely with the 70mm.

How that correlates to the Canon side is another Beer.

50mm is the one ! oops!

I think for the 17-40 you want a 50mm. I use a 70mm for the 16-35.

Thanks for the help, I appreciate it.