Sub Очистить_лист()
nRow = Лист1.Cells(Лист1.Rows.Count, 1).End(xlUp).Row 'в первом столбце последняя не пустая строка
nCol = Лист1.Cells.SpecialCells(xlCellTypeLastCell).Column ' определили номер  последнеи колонки с хоть одной заполненнои ячеикои
For i = 2 To nRow
    Лист1.Cells(i, 20).FormulaR1C1 = "0,3"
    Лист1.Range(Cells(i, 21), Cells(i, nCol)).ClearContents
Next
End Sub

sPath = ThisWorkbook.Path & "files2000"

.Range("B2:B" & lRw).Value = dValue
.Range("C2:D" & lRw).ClearContents

.Range("B2:B" & lRw).Value = 0.3

Sub DataChange()
Dim wBook As Workbook
Dim sPath As String
Dim sFName As String
Dim lRw As Long
Const dValue As Double = 0.3
    With Application: .ScreenUpdating = False: .DisplayAlerts = False: End With
    sPath = ThisWorkbook.Path & "files2000"

    sFName = Dir(sPath & "*.xls*", vbDirectory)

    Do While sFName <> ""
        Set wBook = Workbooks.Open(Filename:=sPath & sFName)

        With wBook
            With .Worksheets(1)
                lRw = .Cells(.Rows.Count, 1).End(xlUp).Row

                If lRw > 1 Then
                    .Range("B2:B" & lRw).Value = dValue
                    .Range("C2:D" & lRw).ClearContents
                    wBook.Save
                End If
            End With

            .Close
        End With

        sFName = Dir
    Loop

    With Application: .ScreenUpdating = True: .DisplayAlerts = True: End With
    MsgBox "OK", 64, ""
End Sub

.Cells(.Rows.Count, 1).End(xlUp).Row - последняя видимая заполненная ячейка столбца А

lRw = .UsedRange.Rows.Count