Automatically Calculate Cell When Changed

There is a column in my spreadsheet which needs to reflect a " percent done".
I used to enter just a numerical value in (eg. 20) to reflect 20% done. My boss wanted it to show as an actual percentage (eg. 20%). I changed the format to Percent, Zero decimal places. Then you had to enter .20 to get the 20% to show.
Now my boss doesn't like that - and wants to be able to enter "20" to get the 20% to show. I put the following code in my SelectionChange event...

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
'other logic
If Target.Column = Asc(PercentDoneColumn) - 64 Then
If Target.Value <> "" Then
Target.Value = Target.Value / 100
End If
End If
'end other logic
End Sub

My problem is that this works well when putting a new percentage in, but not for when you click on a cell that already has a percentage in it. For example, if you have .2 (displaying "20%" in the cell), and click on that cell, it divides that number by 100, and gives you 0.2%, displaying as "0%". I want to enter a number between 0 and 100, have it displayed and stored as a percent. At this point, I'd settle for the displayed, but would prefer to have it stored as a percent, too.
I also want it to not reset my value unless I'm putting a new value in!