In low contrast situations just expose according to the incident light meter.
In contrasty situations, your problems are the highlights. Even using incident light metering, very bright and clears areas might end up burned in the slide. I would consider 2.5 EV above middle grey although I suspect some more room is available in the foot, that is, less and less detail but no white hole (never did exact tests, although I should). My experience refers to Astia 100F and to Sensia 100 in 135. Large format should behave a little better, I presume.
Typical dangerous situation: white surfaces such as marble façades, white buildings, directly lit by the sun.
If you have a spot light meter, measuring the brightest surface you are interesting in, which is very bright, and which you don't want to burn, and opening 2.5 in relation to that, should work.
If you only have an incident light meter, a 1/3 EV or even 1/2 EV closer than your incident light meter says should give you adequate safety margin.
Contrasty situations such as you can have with a marble statue lighted by a grazing light are very tricky as you can have both mistakes, burned highlights and not enough shadows salvaged. In those cases I consider burning a bit of highlights is OK, some details will slide in the foot but the overall appearance will be better. Applying too strictly the "expose for the highlights" in a very contrasty situations might make the shadows suffer too much.
Your tests will permit you to refine these values based also on your development times and development circumstances. I use a rotative processor, 7'30" for all Fujichrome slides, one-shot use, setting my Jobo CPP-2 at 38.3° (chemicals are likely at 38.0°), with four pre-rinse of 30" each at treatment temperature. My one-shot is actually two-shot and the second is at 8'10".
If you have white luminous clouds in the sky don't bother, you will have no much "texture" on them (there is not much anyway) but they will not turn out as a white hole as it can happen with digital. Contrary to popular opinion, slide film has a "foot" although not very extended. Unless the subject are the clouds themselves I would not compensate out of fear of burning the clouds (that supposes you are not in a backlit situation). Again, the highlight will burn but do it "gracefully", without sudden white hole. This allows some margin of manoeuvre.
Slide film is tricky. As an example, in a backlit situation don't hope to have your subject right and the sky behind it right.
I'm planning to begin doing some serious colour negative work to exploit its dynamic range.
Tell us how it goes
Fabrizio
