I need to copy down the contents of a cell the number of cells specified in
an Input Box. I first need to find the last used cell in the column as the
data will keep apending each month. LastRow equals the number in the Input
Box and I can combine that with the Column letter, but I'm having a problem
getting the last used cell address. So if the last cell in the column is M79,
I want to copy the contents down the number of cells in the Input Box. I
hope I've explained this well enough; don't laugh too hard, I'm not very good
at this. Any help would be greatly appreciated as always.

Dim LastRow As Long
Dim FirstRow As Range
Range("M65536").End(xlUp)(1).Select
LastRow = Application.InputBox("How many categories?")
FirstRow = ActiveCell

With Worksheets("Margin %")
.Range("FirstRow:M" & LastRow).FillDown
End With