Walter, I think their reciprocity chart is one of the best things for the film photographer to have in their arsenal, so easy. I have a friend who doesn't have access to the web or anything like that, he pretty much uses HP5+ in 35mm format.
I gave him the formula for HP5+ in very large written format as he is legally blind. I know that he used it successfully for HP5+ for an approximate metered time of 6 seconds on his inbuilt camera meter then switching to manual and the corrected time he had very good success.
I thought about the XP2 factor and wondered if that would work for normal C41 film, maybe not with three and in some instances four layers of different emulsions on the film; but one never knows.
Ein Gutes Neues Jahr to you too!