R1C1 Formula help

Hey,
I am trying to write a formula that calculates percentage change of the values of a column. My problem is that I want to select the last value in the column rather than always a specific row. The number of values in the columns varies. I was wondering how to do that.

The current code is:

    Range("B1").Select
    Selection.End(xlDown).Select
    Range("B15").Select
    ActiveCell.FormulaR1C1 = "=(R[-2]C-R[-13]C)/R[-13]C"

Thanks for your help