-2.5 to +2.5 approx, leave on ISO50 (unless you later determine that you want to do different)
Mid tone in a scene should be mid tone in slide (balanced metering).
Yet do not worry too much of metering and relay on the metering in the Mamiya 7II.
The important is to know and understand how the metering works in the Mamiya 7II, or to be precise where the sensitivity is and that it vary per your focal length! See attached, first one from Mamiya. The second attachment is one that I made based on the first.
For a scene that does not lend itself for balanced metering, then you can use a 1 degree spot meter per the zone system (for slide film) essentially. Dont recall if the Mamiya 7II has spot metering, but that is not 1 degree spot metering in case it has, so be aware and read up what it is. You can of course also use an incident meter e.g. a small Gossen Digisix.
Best regards,
Anders
P.S. Looking back, I wish I had shot all my film images with Mamiya 7II, great camera, sharp optics. Regarding Velvia 50, use if for landsape in what is good photographic light, also details in overcast. If like when I got my first rolls of Velvia years back the colors will blow you away, a magic enhanced rendering of reality. Lovely.