My experience with Velvia is 1.5 to 2 stops max in diffuse illumination. In bright contrasty light it is closer to 1.3 stop. Note that I am speaking as a user of Velvia (100); I last used 100VS in 2004 experimentally and didn't take to the palette. A spot meter should be used over the scene, including mid-tones (the first reading), followed by low and high points, then averaged. The use of a spot meter is to determine individual luminances over specified parts of the scene; an incident will just provide an overall value of the scene with no critical analysis of highlight and shadow. One of the best cameras I've used for transparency film exposures was the Olympus OM4 (1984 to 1988 before switching to Canon T90). As I always teach, this should be more about the film and process of learning and experience through active use, not about cameras and what meters and features and following they have.