18-55 Nikkor + Sea & Sea compact dome

I have the 18-55 Nikkor kit lens behind a Sea & Sea compact dome port. I am using the 40mm extention ring.

There seems a lot of space between the lens front at 18mm and at 55 mm to the dome.

It seems to work fine but would the 22 mm ring be optically better? I understand that the port should be at the nodal point of the lens but how do I find out if I'm using the optimum length ring?

Any advice?

Perhaps this topic should have been put under the DSLR section?

Hi, I would stick with the extension recommended by the manufacturer, does Sea&Sea have a recommendation for this lens?

Sea & Sea list/recommend a flat port for this lens.