Remember that you do not have to extend the time if you open up the lens by the amount specified. So if you need to add a half a stop to an exposure of say, 20 minutes, you don't have to add 10 minutes. You can simply increase the lens opening by 1/2 stop. So if the lens is set to f/8, change it to midway between f/8 and f/5.6. The best is to do tests to see for yourself, and as polyglot says, the film's latitude will also help cover exposure inaccuracy.
Also, at night, there isn't one exact exposure that is "correct". Exposures can range from looking like daytime to definitely looking like night. Personally, I usually don't look to preserve shadow detail so much, so my exposures tend to be shorter. I could let my Pentax LX time a landscape on auto under a partial moon for several hours and it would look similar to daytime, i.e., a "correct" exposure, but I seldom want that.
Also, at night, there isn't one exact exposure that is "correct". Exposures can range from looking like daytime to definitely looking like night. Personally, I usually don't look to preserve shadow detail so much, so my exposures tend to be shorter. I could let my Pentax LX time a landscape on auto under a partial moon for several hours and it would look similar to daytime, i.e., a "correct" exposure, but I seldom want that.