In this code (located in the worksheet code area), I need
to expand things further. As it is now, cells P10 and V10
work with two ranges: U19:U30, and X19:X30, to calculate
values for the "X" range cells. I need to add three more
sets of cells/ranges:

P10, V10, 9:U30 and X19:X30 (existing code)
P11, V11, AD19:AD30 and AG19:AG30 (New. AD = Column 30)
P12, V12, AM19:AM30 and AP19:AP30 (New. AM = Column 39)
P13, V13, AV19:AV30 and AY19:AY30 (New. AY = Column 48)

Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim row As Long
row = Target.row
'Test target column
If Target.Column = 21 Then ' Column "U"
'Test if it is a 'month'
If row = 19 And row <= 30 Then
Cells(row, "X").Value = _
((Target.Value - Range("V10").Value) / _
(Range("P10").Value - Range("V10").Value))
End If
End If
End Sub