Extending the range

In the variation of my earlier UDF, to which your question refers, you can specify the cell to be summed in the function call. Call the function like =Sum3D("B5") to sum up the values in B5 in all sheets between (and including) the sheets specified in A1:B1. You can specify any single cell or even a range of cells.

Function Sum3D(CellAddress As String) As Variant
    ' this function will return -1 if the sheet in Sheet1!A1 doen't exist.
    ' It will return the sum of all sheet from Sheet1!A1 to end if Sheet1!B1 doesn't exist

    ' observe the addressing of "Sheet1" by its CodeName
    ' rather than its name, like in Worksheets("Sheet1").
    ' This has the effect that you can change the name of this
    ' sheet without needing to change the code.

    Dim Fun As Double
    Dim FromWs As String
    Dim ToWs As String
    Dim i As Integer

    Application.Volatile
    FromWs = Sheet1.Cells(1, "A").Value
    ToWs = Sheet1.Cells(1, "B").Value

    On Error GoTo ErrExit
    For i = Worksheets(FromWs).Index To Worksheets.Count
        Fun = Fun + Val(Worksheets(i).Range(CellAddress).Value)
        If Worksheets(i).Name = ToWs Then Exit For
    Next i
    Sum3D = Fun
    Exit Function

ErrExit:
    Sum3D = -1              ' change this to "ERROR" if you prefer
End Function

Note that the earlier function returns an integer which I changed here to become a Variant of Double format. Therefore this function can add up money correctly, for example, or even scientific numbers. In case of error in A1:B1 you can make this function return text, like "Missing Tab" in place of the simple "-1" currently programmed above as inherited from the previous UDF.