I need to loop a formula that references a cell.

A B
1 123 939
2 121 192
3 122 121
4 422 121
....
12 989 121
13 784 623

I need to place a formula in cell A20 (example) that is a
ranking of the items in range("A1:A13"). The problem I
run into is using the r1c1 formula, because the next time
I run the code, the number of rows may be different (i.e.
need to place the formula in cell A5000 and can't simply
use R[-19]C)