I've been messing around - literally wasting film - to nail down long exposure reciprocity adjustments for my fave 2 films - HP5+ and FP4+.
I'm sure this info is available elsewhere, but I've looked loads and can't find it. The ilford chart included in the data sheets only goes up to about 30 seconds as the non-adjusted time, and I was looking for longer than that. And, to be honest, whenever I use that chart, I end up with over exposed negs so I've done a bit of testing. Here are my results.
1, 2 & 4 secs = no adjustment needed.
8 secs = 10 secs
15 secs = 30 secs
1 min = 1 min 50 secs
2 min = 3 min 30 secs
4 min = 11 min 30 secs
8 min = 25 min
16 min = 1 hour 30 min
to be honest, once beyond 5 min actual exposure, accuracy is not crucial. These times were all gained using D 23 developer (1:3) and exposing film at box-speed, based upon a crucial shadow reading reduced by 2 stops (zone III placement if you are into that kind of thing)
