Private Sub Worksheet_Change(ByVal Target As Range)
    Dim rngToCheck As Range
    Dim NumA As Variant, NumB As Variant
    Dim i As Long

    On Error GoTo Whoa

    '~~> Set the relevant range
    Set rngToCheck = Union(Range("G12:G42"), Range("J12:J42"))

    Application.EnableEvents = False

    If Not Intersect(Target, rngToCheck) Is Nothing Then
        For i = 12 To 42 Step 2 '<~~ Loop through only even rows
            NumA = Range("G" & i).Value
            NumB = Range("J" & i).Value

            If IsNumeric(NumA) And IsNumeric(NumB) And NumB <> 0 Then
                If ((NumB - NumA) / NumA) * 100 < 50 Then
                    MsgBox "The Glass U-value and Total System U-value appear very similar. Please check the input values for accuracy!"
                    Exit For
                End If
            End If
        Next i
    End If

Letscontinue:
    Application.EnableEvents = True
    Exit Sub
Whoa:
    MsgBox Err.Description
    Resume Letscontinue
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Value = vbNullString Then Exit Sub
Dim cell As Range

On Error GoTo ws_exit

Application.EnableEvents = False

With Target

If .Column = 3 Then

With .EntireColumn

Set cell = .Find(What:=Target.Value, AFter:=.Cells(1, 1))
If cell.Address = Target.Address Then

Set cell = .FindNext()
End If

If Not cell.Address = Target.Address Then

MsgBox "This Glazing Reference already exists. Please ensure you have a unique reference identifier less than 20 characters in length", vbOKOnly
End If
End With
End If
End With

ws_exit:
Application.EnableEvents = True
End Sub