I'm not sure if anyone makes one, but it would make sense.
Originally Posted by Alan Klein
I remember reading. though, and forgot to mention, that not only is there a K factor, but also a C factor and what value the manufacturer uses seems to be up to them. I believe the 18% gray came from old print/publishing. If I recall correctly, the reason Ansel Adams and others pushed for 18% had to do with being able to print their negatives and have them published at the correct levels.
You may want to check a printing house for a 12% card. I just test using my meter and go with that. I've found with my Starlite 2, I get box speed for FP4+. The difference between 12% and 18% is about 2/3 stop, I think. I'm sure your meter can be checked by shooting 2 shots of the same gray card 2/3 stop apart and printing it out. I never tried it, so it's just a theory.