I am currently using a Hasselblad 500c with a Kiev TTL metered prism (yes, i am poor). I just started using it and I wanted to calibrate it before I go out and start shooting. I have a minolta IV flash meter and a Canon 5dmkii to make metering comparisons. Now I don't have a grey card, would I specifically need one if I wanted to calibrate the meter? Or is there any other way for me to do this?