Velvia 50 is more commonly re-rated at EI40 or EI32 when it is in the small 35mm format.
It is much easier to meter RVP50 in medium and large format than it is to rely on an onboard camera meter. This is why so many, many Ilfochrome Classic images were produced from anything other than 35mm, because it did not present or compound contrast control problems amongst small-format pjhotographers who had a poor understanding of using Velvia in specific lighting, as opposed to larger format photographers who could meter Velvia in any type of light and come away with a serviceable image. All of the Velvia emulsions had this contrast problem, including but not limited to the ugly 100F and also the hyper-enriched red channel 100 emulsion.
Re-rating at EI32 will significantly increase the risk of highlight clipping and mushy shadows — once they've gone, they are gone. There is no bringing them back, even with a proverbial magic wand! Essentially re-rating is a hobbling compromise with no real gain. Exposure is also something to think about. In 35mm, half-stop exposure is much better than third-stop. Problems announce themselves when an exposure is given 0.6 stops when it really requires no more than 0.5, resulting in either too dark or too light. It may not look much of a figure on paper, but to Velvia it is the tipping point. There is a fine balance to be struck.