Does anyone know exactly how to calculate the exposure time, taking into account reciprocity failure? For example, say I happen to have Ilford Pan F, ISO 50. I determine that my exposure time should be 30 second with out factoring in reciprocity. When I look at the Ilford fact sheet reciprocity chart (here) it shows that I actually need to expose for 150 second.

This seems like it could be fine for the work that I want to do--mainly night scenes near city lights.

If I wanted to do as the OP is suggesting, should I just choose a better film with less (lower?) reciprocity failure, or should is there a way to calculate it?

I have never done an exposure longer than say 2 or 4 seconds, so forgive me for any naivete.