Une proposition :
Sub TestCalculHeures()
Dim I As Integer, MesLignes As Variant
MesLignes = Array(8, 12, 16, 20, 24, 30)
For I = LBound(MesLignes) To UBound(MesLignes)
Range(Cells(MesLignes(I), 3), Cells(MesLignes(I), 3 + 6)).Formula = "=IFERROR(C" & MesLignes(I) - 2 & "-C" & MesLignes(I) - 3 & ",0)"
Range(Cells(MesLignes(I), 12), Cells(MesLignes(I), 12 + 6)).Formula = "=IFERROR(L" & MesLignes(I) - 2 & "-L" & MesLignes(I) - 3 & ",0)"
Next I
End Sub