dynamic range formula

here is some code you can use / customize. The code identify the first row that does not have an F in column C.

Sub test()

Dim rCell As Variant
Dim lRow As Integer
Dim rng As String


Range("c4").Select
rCell = "F"

Do Until rCell <> "F"
    
    
    rCell = ActiveCell.Value
    
    ActiveCell.Offset(1, 0).Select
    
    rCell = ActiveCell.Value
    

Loop


lRow = ActiveCell.Offset(-1, 0).Row

rng = Range("a1", Cells(lRow, 4)).Address


MsgBox rng


End Sub