Sigma 50mm Macro

After reading numerous reviews and viewing tons of excellent photography taken with the Sigma 50mm f/2.8 EX DG lens, I have chosen to purchase this lens for my macro/fish portrait photography on my Canon 20D. There has been some mention on its close working distance in order to achieve 1:1 but I have yet to find the exact distance required behind a flat port. Does someone know the distance?

This is just a crazy idea but can a teleconverter be used with this lens? Sigma mentions you can not use their teleconverters but I'm wondering if any other would work.

188mm from subject to CCD.

Approx 40mm from the end of the lens to the subject.

I guess It all depends which housing and which flat port.

I use it a lot , now instead of the Nikon 60mm. I have used it with a Kenko 2x TC.

Thanks for the info. I appreciate it.