Somewhere around here, if it's survived the migration to the new software, I've posted a magnification/exposure table as an MS Word doc file (but such tables can be found in any book on macro photography).

What I do with any format camera is put a ruler in the scene at the subject position or estimate the width of the subject field and compare it to the width of the film, so if I'm shooting a macro scene that's about 4.5" wide with a 2-1/4" square (6x6cm) camera, then my magnification ratio is 1:2. Then I look up the exposure factor on my table and make the correction--1-1/3 stops in this example. No need to think about focal length, lens extension or subject distance with this method--just magnification ratio.

For DOF--you'll usually need all you can get. Just stop down all the way and use multiple flash pops or very long exposures if necessary. There are various DOF calculators on the net. One that I like is called "f/calc" which is a shareware program that can do several photographic calculations.