simple, the rule applies to the grand parents as well.
So you refuse to answer the question.
I will extrapolate for you then.
The person with three 100% African grands is only 75% African. So now although they are Black, they are not 100% African so they can't contribute to their grandkids Blackness. Actually, I would guess most Black people in America don't have three 100% African grands, so the population of Black people is actually very low by your math.
If all this shyt sounds retarded
IT'S CAUSE IT IS!!
There's no formula that you can propose that will make any sense besides:
If you are descendant from Africans in any way and you are down then you are Black.
The brother in the OP is more of a Black man than all of us if we're going by deeds.