'Код в модуле листа
Private Sub Worksheet_Change(ByVal Target As Range)
    If IsFormLoaded("UserForm1") Then
        If Not Intersect(Target, Range("A1")) Is Nothing Then
            If IsNumeric(Target) Then UserForm1.TextBox1.Text = (Range("A1").Value * 100) & "%"
        End If
    End If
End Sub

Private Function IsFormLoaded(formName As String) As Boolean
    Dim frm As Object
        For Each frm In VBA.UserForms
            If frm.Name = formName Then
                IsFormLoaded = True
                Exit Function
            End If
        Next frm
    IsFormLoaded = False
End Function

'Код в Module1
Sub Кнопка1_Щелчок()
    UserForm1.Show 0
    If IsNumeric(Range("A1")) Then UserForm1.TextBox1.Text = (Range("A1").Value * 100) & "%"
End Sub