I just did a test with my grey card and the Gossen would underexpose by about half a stop using reflective vs incident, but that may just be my grey card!
Just a little update:
THE DOME FOR THE LUNA STAR F2 WILL FIT THE GOSSEN DIGIPRO F.
I ordered the dome from B&H and it came in the other day. Snapped onto my Digipro and everything seems to be working fine. Just thought I would update the thread in cas anyone in the future has the same problem :)
If you look on the Gossen web site at the small parts section and fill in the request form with what you want they will send you one.
Meters, generally are not calibrated to 18%. There really is no "industry" standard as we are lead to believe. That's why Ansel Adams and Fred Archer developed the zone system.