Your calculations are correct, provided (as already pointed out) that there was no further pedigree collapse.
If there is no collapse at all, you have 2^n ancestors at generation n, where 2^n is 2 raised to the nth power: 2^1 = 2, 2^2 = 2*2 = 4, 2^3 = 2*2*2 = 8, and so on.
If your grandparents were first cousins, then one pair of their grandparents were identical: the same people. That is, a pair of your great-great-grandparents (four generations back) overlapped. So you had 2^4 - 2 = 16 - 2 = 14 gggparents.
Taking this further, because the whole trees overlapped, you had 2^5 - 2^2 = 32 - 4 = 28 ggggparents, 2^6 - 2^3 = 64 - 8 = 56 gggggparents and so on.
The general formula is that you had
- 2^n ancestors at generation n above you for n <= 3
- 2^n - 2^(n-3) ancestors at generation n above you for n > 3.
You could express 2^n - 2^(n-3) = 8 * (2^(n-3)) - 2^(n-3) more simply as 7 * (2^(n-3)). If you calculate this for n = 4, 5, 6, 7 you get 14, 28, 56, 112, just as you found.
The sequence continues doubling each time: 224, 448, 896, 1792. But as Dennis and Daniel have said, there was almost certainly more collapse as long ago as that, so the true numbers would be lower.
