Your question is actually simple, but may seem complicated. Both meters are trying to tell you what exposure would be needed to render 18% gray corectly. One is most likely reading through the lens seeing a small area where as the other will be reading the entire scene. One way around this is to get a " gray" card and work from what the meter reads from it. The other is to us your hand ( if your "white" or asian it will give you a pretty close exposure. And remember if its worth getting the kit out it's worth bracketing the exposure as a back up. With negs one over one center and one under, with slides that's 1/2 stops between.