As mentioned in episode 16 of The Art Of Natural Beauty, below I have provided a copy of the charts I use to determine proper exposure times, based on the reciprocity failure of the given films I use. You can simply right click and save these charts to print out and keep in your camera bag as I do.
Just a brief recap on what reciprocity is and the resulting breakdown of the law of reciprocity, known as reciprocity failure. Reciprocity is a law, or formula, that implies that aperture and shutter speed are an inversely proportional relationship between the intensity and duration of light that determines the reaction of light-sensitive material. As you decrease the size of your aperture, f/5.6 to f/16, you are letting in less light and thus you need to increase your shutter speed to allow the light to expose your film for a longer duration, in order to get the correct exposure.
Reciprocity failure occurs when the normal rules of reciprocity no longer apply. This happens when your shutter speed increases beyond the ability to compensate or counter with aperture. When the normal reciprocity time of a given film is exceeded a breakdown in the relationship occurs and therefor must be compensated by adding extra time to the indicated expose time provided by your light meter. When less light is available, the silver halide crystals in the film are not evenly struck by photons of light, and the density of the resulting exposure is lowered. Each film is different and some are more forgiving than others.
Below as I have stated are two charts for correcting for reciprocity failure in Fuji Velvia and Kodak Ektar 100 films. As the charts suggest, each film is different, however by following the indicated vs corrected exposure times listed, you can correct the exposure and achieve the desired outcome nedded for proper exposure.