pulpo

I purchased a Nikon D300 last October and I have been able to get underwater with my gear a few times in the last year. I'm using a Subal housing and have recently purchased a 2x TC to go with my 105 VR in hopes of practicing super macro. Does anyone use this setup, and if so, what length extension ring are you using? The TC is Kenko if that makes a difference. I purchased a used 40mm thinking that I had the right on. I am about 1/8" shy. Any help would be greatly appreciated.