Patrick Gainer established his reciprocity formula based on data from this article:
Ilford hasn't changed it's reciprocity data for HP-5 for decades, but the film has changed. Here's a quote from Bond's article concerning HP5+.
You'd likely do much better with Bond's recommendations for reciprocity adjustment for HP5+ than with the data sheet from Ilford.Ilford curve outdated, too?
In publications dated September 2002 (sent to me in January 2003), Ilford still had a single old curve for correcting reciprocity departure for HP-5+ and 100 Delta. I found the behavior of these films to be quite different from the curve, and also different from one another.