What length of bellow would you need for 12x20

[windrider]

If I were to get into a diy build on a 12x20 with say a 355mm g-claron, what length of bellows would I need for such a large format If I wanted shoulder/headshots? Just tinkering with some ideas but im a little lost on how to figure out the length needed for something so far past infinity.

[VPooler]

Find out the bellows draw required for the focal lenght at distance.
http://www.kenleegallery.com/html/tech/bellows.php
Should be over half a meter for headshots!

[windrider]

Perfect!!!! Thanks.