You'll want to test it. A rule of thumb for transparencies (and something that has worked for me) is that slight underexposure is good for drama and exposing at box speed is best for accuracy. On the flip side equipment and technique varies and what the manufacture calls iso 100 might be something different for your equipment or your metering style, and or shutter(s) might have variances. FWIW I always bracket with slide film and have never shot velvia 100 (I also rate velvia 50 @ 40 as a slide and when xprocessing).