Alex,

Since you said that you were doing still lifes, I would imagine that you may be into bellows extension compensation territory. I have had that screw up more then a few exposures.

Additionally the fstop will need to be considered. As degree of magnification increases the need for smaller fstops (to hold depth of field) increases as well.

What I would do in this situation would be to set up the camera and using a strong light be sure that my subject was in focus. Next I would measure bellows extension. Next I would measure ambient light at the subject level. Next I would calculate my exposure. Next I would add my bellows extension factor. Then I would calculate reciprocity for that exposure. That would then be my exposure. Good luck.