Reciprocity effects are probably more important when shooting colour at night. With B&W I would tend to experiment and keep notes. To avoid reciprocity problems, it is often suggested that you use a set speed and vary aperture. This is not very helpful if you are making images which require front to back focus. I would tend to use an aperture of f8-16 and try to calculate a base exposure time. You can then bracket plus one or two stops. There are some books on low light photography that contain useful exposure charts for various subjects. You could also try a faster film. I have had good results with Delta100 in 35mm for night work. If you are using 120 you could go to 400ASA. Get a locking cable release and a stopwatch to help with long exposures. I use a keyring LED torch to see what I'm doing. One approach that has worked for me with urban night images is simply using aperture priority auto and letting the camera choose the speed. It's not very scientific, but it has produced some usable negatives. Alex