Bruce,

I've used Ilford films for ages and they print a reciprocity graph in their film brochures. It's the same graph for each of the films - I don't know why as I'd have thought different speed films would have had differing reciprocity characteristics. It's possible to either make out a table of figures from this rather than having to carry round the graph (for the most common speeds you use) or even to do a bit of measuring and put the figures onto a spreadsheet so that you can calculate the equation of the curve and then work out whatever correction time you want. I have to agree with the other posts that, at longer times, it's not an exact science and plus or minus a reasonable amount doesn't make much of a difference.

I tend to measure the exposure I want, add on the reciprocity correction and then bracket from that figure - it means you get something on the negative although the times can get very long on the third bracket at +2 stops (no point bracketing below the time for b&w negs as you start losing shadow detail). e.g. measured time = 30 sec, add on correction to give 2 min 40 sec, expose at 2min 40sec, 5min 20sec, 10min 40sec. The problem with this is highlights in the picture will be way overexposed if you're trying to get in a lot of shadow detail and you have todevelop & print the negs very flat to kill the contrast.