It depends on the approach you're taking. If I tell you that film A needs three stops of compensation at 30 seconds exposure you can do one of two things. You can open up the aperture three f-stops and you're fine. But if you try to double the exposure three times, you're going to get compound reciprocity failure because at the adjusted exposure of 240 seconds, you've got additional reciprocity to deal with, not just the adjustment for 30 seconds. You can calculate that adjustment with one of several formulae for adjusting for reciprocity with exposure time (as opposed to adjusting with lens aperture).
Originally Posted by holmburgers
There's a marathon thread on a lot of this here: http://www.apug.org/forums/forum37/1...sbehavior.html and there have been subsequent threads as well.
The Howard Bond article discussed there is currently available at: http://www.willwilson.com/articles/0...ciprocity2.pdf
Most manufacturers reciprocity data for B&W film (especially Kodak and Ilford) has not changed in the last 30-40 years, while the films have changed a lot. See the Bond article for comments on that.