I have a 10 degree reflective and incident meter that can average three readings. How do you do it?
No mystery there, although 10 degrees is very wide for a spot meter and overlap of adjacent tones could throw off straight and averaged measurements.
With RVP50 I [1 degree] spot between 2 and 8 (most commonly 4), adding a mid-tone either from the within the scene or a grey card at the end, and averaging. But a mid-tone is not strictly necessary if you are going to straight average, but small differences can be seen in films like Velvia in a mix of shadows and illuminated areas. There are a few customisations to my meter which take measurements and averaging for a more technical level, mostly to provide for a slight 'key' that is required for hybrid printing (where some illumination from the scene is lost).
Outback Australia, where the colours and contrast are very, very intense during the day, in the morning and again in the evening as the light begins to soften (granite boulders, plateau, caves, ghost gums, etc.)
[Sirius, take note of this!] is the only place really where I would re-rate RVP50 at EI40, or consider switching to Provia.
None of my metering includes any part of the sky unless there are elements in it critical to aesthetics or complimentary to the main image e.g. afterglow in the evening sky is metered, so too is early dawn light, as in both instances the light is reflected on the landscape (especially water).
B&W is much less critical to meter than transparency film, for obvious reasons of generous latitude, but care with tones and spectral influences must still be considered.