In general, we don't go around labeling black people by what percentage black they are. Black people as a population have varying skin tons from very dark to very light - they're still just labeled as "black." I've never heard a black person call themselves "three quarters black" or "seven eights black."

While you do hear people call themselves "half X and half Y," that has to do with their personal heritage and history. As far as the rest of society looks at someone, if they're black, they're "black." Doesn't matter what percentage. That's why so many half black half white people associate as "black" rather than as "white" - because that's how society labels them and treats them, so that's why they identify as because they've lived the black experience rather than the white experience as far as interacting with society goes.