Ok, I distill from your answers that if I want to take a measure of the ratio, I can use method C (separate measure with the dome at each side of the object, pointed at the camera) and it will work just as if I had used method B (separate measure with the flat thingie pointed toward the light source).

I can ignore the existence of the flat thingie, and live happy with my incident meter which only has the domed thingie.

Many thanks.

Fabrizio

PS Stillsilver wanted to give me this answer I guess but inverted method C and method B. Otherwise it is clear. I'll do as Mark Barendt says. I just use method A for average exposure, and C for checking ratios, and that's all. And yes, I can skew final exposure toward one of the two readings with method C if I want to expose in a non-average way.