Theres actually a easier way than copy and pasting. In the bottom right corner you can mouse over and you'll see a sort of plus sign appear. Click and drag that and it will copy the formula in each cell but with the correct cell number for the formula. So if you drag up it will say A12, A11, A10, etc, etc so you don't have to go back and change it.
You'll see what I mean if you try it, its kind of hard to explain.