Trying to calibrate exposure, film and dev with meter and wedge step:

I thought I already understood this well but now I'm getting confusing results.


Stoufer RZ1 (with zones)
Weston Master V meter with diffusion cone
Kodak 18%

The Kodak 18% grey matches zone V on the Stouffer step wedge. As I thought.

If I take an incident reading with the cone on I get exposure value 8.

Then if I take a reflective reading using the card, I get a full stop difference at exposure value 9.

I have assumed for years that incident readings taken with my meter would give an exposure for equivalent of a zone V card reflective reading. Am I wrong?

Which is the true mid-grey (V) exposure?