Contax 645 extension tube: which size?

I own a Contax 645 with an 80 mm lens. I'd like to do some portrait work, and I anticipate purchasing a 140mm lens and an extension tube to allow close focusing for tight face shots. Would the 13mm tube be the right length for this? They're so expensive that I can't afford to get the wrong one. I'd appreciate hearing from anyone who has used this set up.

Thanks -

Dave

For my Bronica 645, I have never had need for anything beyond the shortest extension tube (14mm) for my 150mm to take tight head shots with it.