Digital cameras make BAD exposure meters for film cameras, triply so for night exposures as digital doesn't experience reciprocity the way film does. As mentioned, first find the reciprocity charts for the film(s) you are using. Then if you have a spot meter, meter the same way you would for a daytime exposure- pick a spot that you want to render middle gray in the final print, and meter off of it. Use that as your base setting, then adjust for reciprocity. When in doubt, if you can't find a reciprocity chart, 1-10 seconds on the meter= +1 stop. 10-30 seconds, add two stops. Beyond 30 seconds, add three. When developing your film, remember to compensate in reverse - your highlights will be proportionately blown out to the degree that you adjusted for reciprocity, so for each additional stop you gave, cut development by 10% or you'll be guaranteed to have highlights completely resistant to burning in or inky blobs for shadows and midtones, and exposure times for each print into the range of reciprocity failure for your paper (and that's an achievement!).