I think the thicker old style emulsions exhibit this because of a 'threshold' phenomena, where below a certain amount of light, the light senstive compounds in the emulsion are just not activated. My rule of thumb is [indicated->actual] [2->4],[5->20],[10->50],[30->3-1/2m] for old style films. Your mileage may vary depending on the developer and the film type. Now when I am in those situations, I fall back on the Phil Davis BTZS palm pilot expodev program's recommendation, and so far, it has been right on the money.

When you are in situations with long bellows extensions, reciprocity problems and filter factors, I have found that little program to be invaluable. At first I felt like a wimp using this 'crutch'. But then when my negs kept coming out of the wash exactly like I wanted, I decided to just be a wimp and make my life easier when it comes to printing. And anyway, is it any more unmanly than setting your 35mm to 'A' and then shooting away? Definition of a reasonable man: "A reasonable man can find a reason for anything".