Just to chime in, if you are using the in-camera meter there is no reason to adjust for the filter factor, at least in theory. When using an external meter tean lowering the ISO is called for. Each ISO designation is a 1/3 stop increment. Using a lower ISO will increase the exposure called for by 1/3 stop. The 1/3 stop convention was determined to be the least noticeable significant exposure change one can make. This was more important to those exposing chromes. If you have difficulty finding the third stop point on the aperture ring, or on those few older lenses with half stop clicks, the 1/2 stop point will do. The difference between 1/3 and 1/2 stop is negligible, especially when shooting negative film. Also my rule of thumb for shooting negative film is to err on the side of overexposure.