@wiltw checked and you’re right. I remember the Velvia advice was “40” but I also remember the generalized advice to decrease exposure slightly for better saturation. Something sounded wrong. The two recommendations contradict each other. So we need to give context.
They are both correct memories of the advice given by Galen Rowell in Outdoor Photographer magazine. He was talking about two different films.
In context he wrote “I have found it (Velvia 50) i...nstead of shooting it at ISO 64 as I do with Fuji Pro 50 to get richer tones from slight underexposure, I set my camera right on ISO 50 for rich colors and on ISO 40 for more open shadows.
So there you have it: Velvia at 50 or 40 depending…
This has been bothering me for years so glad it’s clear now.