I'm looking to do this same thing (copy formula down to the length
of data in an adjacent column) but with vba. I know how to use 'Range
(ActiveCell, ActiveCell.End(xlDown)).Rows.Count' to get the number
of rows but can anyone tell me the code to fill  the range  with a
formula?

For example, column  A contains a range of data that could be any
lenght from 10 rows to maybe 3000 rows long. I want to use vba to
insert a formula in column B and have it automatically extend to the
length of the data in column A. How is this done?