I got my first ever roll of velvia back today taken with a Fuji G617. First ever roll of slide film for that matter. While one of the shots was spot on the others were overexposed, but only slightly according to the lab apparently. The advice I got given by my tutor was to rate it ISO 25 on cloudy days and ISO 40 on a bright days. Is this sound advice? It seems a bit of a rough guide, how do you differentiate inbetween. Bearing in mind I also compensated by a stop for the ND center filter. Any tips for getting right results would be helpful.