I'm looking for a way to autofill my formula down to the last row in the dataset (which is variable) using a range which is also variable. I have highlighted my issue below at the bottom.
Here is the code that I have now:
Sub MissingData()
    Dim LastRow As Long
    Dim LastCol As Long
    Set ws = Worksheets("Insert Data")
    With ws
        Last Row = .Cells(.Rows.Count, 1).End(xlUp).Row
        Last Col = .Cells(1, .Columns.Count).End(xlToLeft).Column
        'Inserting Column Header next to the last column in the data set in row 1"
        .Cells(1, LastCol + 1).Value = "Header"
        'Inserting Formula next ot the last column in the data set in row 2"
        .Cells(2, LastCol + 1).Formula = "=iferror(AJ2,""YES"")"
    End With
    Dim FoundCell As Range
    'Looking for the Last Row in the Dataset"
    'Column A:A will always be populated with data and will be the driver
    'for how many rows are in the data set"
    LR = Worksheets("Insert Data").Range("A:A").End(xlDown).Row
    With ws
        'I set this and then called it using select because my range above
        'and the location of this cell could be variable"
        Set FoundCell = .Cells(2, LastCol + 1)
        FoundCell.Select
        'Here lies my issue.  Using this syntax the formula is filled all the way
        'to the last row available in Excel which is like 1 million something.
        'I just need it filled to the last now that i set above"
        Selection.AutoFill Destination:=Range(ActiveCell, ActiveCell.End(xlDown))
    End With
End Sub
 
    