You can't use data validation on a cell with a formula. You could use a worksheet macro though. Here's one you can try. I assume the cell with the sum formula is A10 on this worksheet. Change this in the code to fit your sheet. Also I assume the sum cell and its precedent cells are formatted as %.
To install the code:
Open the file to the worksheet with your sum formula
Right-click the sheet name tab and choose 'View Code'
Copy the code below and paste it into the white space in the VB editor window that has opened.
Save the workbook. If you are using Excel 2007 or 2010, you must save it with a .xlsm (not .xlsx) file extension or the code will be removed when you close the workbook.
Private Sub Worksheet_Change(ByVal Target As Range)
If Range("A10").Value > 1 Then
Dim adr As String
adr = Range("A10").Formula
adr = Mid(adr, InStr(1, adr, "(") + 1, InStr(1, adr, ")") - 1 - InStr(1, adr, "("))
If Not Intersect(Target, Range(adr)) Is Nothing Then
MsgBox "The selected range must sum to <= 100%"