Sub TestIt()
Const c_START As String = "P2"
Const c_STEP As Long = 20
Const c_STOP As String = "P41"
Dim rngBlock As Range, strStop As String
strStop = c_STOP
On Error Resume Next
Set rngBlock = Range(strStop)
If rngBlock Is Nothing Then strStop = _
Cells(Rows.Count, Range(c_START).Column).End(xlUp).Address
On Error GoTo 0
Set rngBlock = ActiveSheet.Range(c_START).Resize(c_STEP)
Do While rngBlock.Cells(1).Row < Range(strStop).Row
rngBlock.Cells(1).Autofill rngBlock, 0
Set rngBlock = rngBlock.Offset(c_STEP)
Loop
End Sub