Use Column Header Names Instead of Column Letters in Formula in a Macro

Hello,
The code below was in response to this question:

I would like to compute correlation matrices (using CORRELATE) for each group in a worksheet. The correlation matrices should be by group (5,6,7,12,13, or 14) on columns D - H.  Ideally the new sheet would be named with at least the group number.

The fact that it only works on D - H, is hanging me up.  Instead of using the "Column Letters" D - H, I'd like to insert the column header names (i.e. those in row 1).  

I have attached an example worksheet showing the 'data' and example output (e.g. CORREL on Columns D - H on Group 5).  I have been asking a few questions on this topic. I am trying to get at the core idea of how to make the macro more adaptable. I hope this is clear and explains my goal.

I thought if I could enter the column names then I could easily change what data is included in the CORREL formula within the macro.

Please advise,
Thanks,
Cheers,
JE

previous questions:
https://www.experts-exchange.com/Software/Office_Productivity/Office_Suites/MS_Office/Excel/Q_27040912.html
https://www.experts-exchange.com/Software/Office_Productivity/Office_Suites/MS_Office/Excel/Q_27025172.html

Sub MakeMatrix()
    
    Dim arr() As Long
    Dim CurrentGroup As Long
    Dim SourceWs As Worksheet
    Dim LastR As Long
    Dim Counter As Long
    Dim DestWs As Worksheet
    
    Set SourceWs = ThisWorkbook.Worksheets("data")
    With SourceWs
        .[a1].Sort Key1:=.[a1], Order1:=xlAscending, Header:=xlYes
        LastR = .Cells(.Rows.Count, 1).End(xlUp).Row
        CurrentGroup = .Cells(2, 1)
        ReDim arr(1 To 3, 1 To 1) As Long
        arr(1, 1) = CurrentGroup
        arr(2, 1) = 2
        For Counter = 3 To LastR
            If .Cells(Counter, 1) <> CurrentGroup Then
                arr(3, UBound(arr, 2)) = Counter - 1
                ReDim Preserve arr(1 To 3, 1 To 1 + UBound(arr, 2)) As Long
                arr(1, UBound(arr, 2)) = .Cells(Counter, 1)
                arr(2, UBound(arr, 2)) = Counter
                CurrentGroup = .Cells(Counter, 1)
            ElseIf Counter = LastR Then
                arr(3, UBound(arr, 2)) = Counter
            End If
        Next
    End With
    
    For Counter = 1 To UBound(arr, 2)
        Set DestWs = ThisWorkbook.Worksheets.Add
        With DestWs
            .Name = "Group " & arr(1, Counter) & " Matrix"
           .[a2:a6] = Application.Transpose(SourceWs.[d1:h1])
           .[b1:f1].Value = SourceWs.[d1:h1].Value
           .Range("b2, c3, d4, e5, f6") = 1
           .[b3].Formula = "=CORREL(Data!D" & arr(2, Counter) & ":D" & arr(3, Counter) & _
                ",Data!E" & arr(2, Counter) & ":E" & arr(3, Counter) & ")"
           .[b4].Formula = "=CORREL(Data!D" & arr(2, Counter) & ":D" & arr(3, Counter) & _
                ",Data!F" & arr(2, Counter) & ":F" & arr(3, Counter) & ")"
           .[b5].Formula = "=CORREL(Data!D" & arr(2, Counter) & ":D" & arr(3, Counter) & _
                ",Data!G" & arr(2, Counter) & ":G" & arr(3, Counter) & ")"
           .[b6].Formula = "=CORREL(Data!D" & arr(2, Counter) & ":D" & arr(3, Counter) & _
                ",Data!H" & arr(2, Counter) & ":H" & arr(3, Counter) & ")"
           .[c4].Formula = "=CORREL(Data!E" & arr(2, Counter) & ":E" & arr(3, Counter) & _
                ",Data!F" & arr(2, Counter) & ":F" & arr(3, Counter) & ")"
           .[c5].Formula = "=CORREL(Data!E" & arr(2, Counter) & ":E" & arr(3, Counter) & _
                ",Data!G" & arr(2, Counter) & ":G" & arr(3, Counter) & ")"
           .[c6].Formula = "=CORREL(Data!E" & arr(2, Counter) & ":E" & arr(3, Counter) & _
                ",Data!H" & arr(2, Counter) & ":H" & arr(3, Counter) & ")"
           .[d5].Formula = "=CORREL(Data!F" & arr(2, Counter) & ":F" & arr(3, Counter) & _
                ",Data!G" & arr(2, Counter) & ":G" & arr(3, Counter) & ")"
           .[d6].Formula = "=CORREL(Data!F" & arr(2, Counter) & ":F" & arr(3, Counter) & _
                ",Data!H" & arr(2, Counter) & ":H" & arr(3, Counter) & ")"
           .[e6].Formula = "=CORREL(Data!G" & arr(2, Counter) & ":G" & arr(3, Counter) & _
                ",Data!H" & arr(2, Counter) & ":H" & arr(3, Counter) & ")"
        End With
    Next
    
    MsgBox "Done"
    
End Sub

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JE-EE-corr-matrix-by-group.xlsx