Metering for Color Reversal
Metering for color reversal film is a two stage process: first for exposure, second for development. If you are shooting an entire roll under controlled lighting conditions, you don't need to nail the exposure. If you are within the film's latitude and have some accurate lighting readings you can set the final speed by push processing.
Push Processing Color Reversal
Color negative film cannot be pull or push processed because the development time is calibrated for a balanced development of the dies in all three layers. With color negative film, variation in exposure results in a negative that can range from thin to dense.
The development time for the Color Developer is also fixed for color reversal, but the First Develper is B&W chemistry that can be push processed. Calculating the development time for the B&W mask is what gives you the ability to rate Fuji color reversal film from -½ to +2 stops.
The following diagram shows eight incident readings I took for a portrait session. For each reading I turned 45°
From left to right there is a 1½ stop difference. I will meter for highlights by setting the aperture at f/8, but before development I have to determine how to rate this film in development speed.
Color negative is similar to digital RAW in that we can set the "gain" later. Color reversal is not like this—this photograph is rendered at development time. The positioning of the subject, placement of light sources are combined with development to create the final photograph.
The instructions that come with the Arista E-6 Developing Kit list the development times for push processing by one or two stops, but we can approximate the development times for any step inbetween.
If you consider this to be a complete picture of development latitude then it makes sense to rate Provia 100 in the range of 125 to 250 so that there is always some room for fine-tuning development.