Get Last Populated Row In A Range

My VBA is very rusty, and I can't get the following code to work. I need to workout the number of rows that contain data in a range so that I can eventually write a formula in a adjacent cell, but I keep getting errors (Type mismatch or Subscript out of range) at the first step!

Code:

Sub create_product_column()

Dim num_rows As Range ' The number of rows in the datarange
Dim my_workbook As workbook ' The name of this workbook
Dim my_worksheet As worksheet ' The name of the worksheet to work on

[Code]....