Insert Formula In Dynamic Range

I am trying to insert a formula into the cells in a column dynamic range.

I never know how long the column will be from instance to instance.

I have been trying to use the following code,

Sub insertf()
'
' insertf Macro
' Macro recorded 10/11/2007 by bbactadmin
'

'
Do Until myrange.Value = ""
ActiveCell.Range("f10").Select
ActiveCell.Offset(1, 0).Range("f1").Select
Range.Formula = _
"=VLOOKUP(LEFT(E11,3),fam,2,FALSE)"
ActiveCell.Offset(1, 0).Range("f1").Select
Loop
End Sub

I am trying to insert the lookup in all of the cells in the column within the dynamic range.