I remember reading that Canon and Nikon use 12.5 for their K, which means with c=250, they come out at around 15.7%.
Metering an exterior scene using a disk diffuser requires averaging two measurements. This means using it's value of C is inappropriate to determine a value for average reflectance. You need to use the semihemispheric diffuser. There's a detail explanation in the thread "Is the K factor relevant to me or should I cancel it out?"