Paste the function below into a standard module.

Open your form in design view, right-click the text box where you want the
value to be interpreted as a percent, and choose Properties. On the Event
tab of the properties box, enter:
   =MakePercent([Text1])
replacing "Text1" with the actual name of your text box.

The code examines the text in the field. If the user typed a percent sign,
it leaves the value alone, because Access will interpret that correctly. If
not, it will divide by 100.

Public Function MakePercent(txt As TextBox)
On Error GoTo Err_Handler
   'Purpose:   Divide the value by 100 if no percent sign found.
   'Usage:     Set the After Update property of a text box named Text23 to:
   '               =MakePercent([Text23])

   If Not IsNull(txt) Then
       If InStr(txt.Text, "%") = 0 Then
           txt = txt / 100
       End If
   End If

Exit_Handler:
   Exit Function

Err_Handler:
   If Err.Number <> 2185 Then  'No Text property unless control has focus.
       MsgBox "Error " & Err.Number & " - " & Err.Description
   End If
   Resume Exit_Handler
End Function