Macro to Limit % - vba

Question

I have an excel spreadsheet which has multiple tabs and cells filled with % values. There are some cells which read 20%, 30%, 40% etc and the format of these cells are 'percentage'. There is a sum cell which sums up all these percentages to equal 100%. I am trying to write a simple macro/rule which would constantly read the sum cell and throw out an alert if it does not equal 100%. I am not sure how to proceed.
I am trying to use Data Validation to do this instead of any coding as what I really want is for the users to be unable to change the values of cells if the total changes from 100. Data Validation still is not working. It always seems to throw errors. Anyone have any thoughts on this ? I have selected the range of numbers and in data validation I am choosing Custom where I am saying = Sum(C5:C10)=100. This is always throwing an error even when the total is 100. Any thoughts ?

Answer 1

If I've understood your question correctly you'll be able to do this without any code.
For example, your percentage cells are on Sheet3, Sheet4 and Sheet5 in the ranges B2:B6.
On Sheet6 you can use the formula =SUM(Sheet3:Sheet5!$B$2:$B$6)
NB: This is a 3D formula and works providing Sheet4 is between Sheet3 & Sheet5 in tab order.
You can then use conditional formatting on the total - Format only cells that contain - Cell Value is greater than 1.
Then format it as something obvious (I generally use bold yellow text with a red background).
Ok, as reply to your comment, add this code to the worksheet code module which will run every time the sheet calculates:
Private Sub Worksheet_Calculate()
If Cells(6, 3) > 1 Then
MsgBox "Total is too high"
End If
End Sub

Answer 2

I would not use the Worksheet_Calculate event procedure because doing so would cause much too frequent evaluation of the MsgBox condition.
Try to keep that to an absolute minimum like so:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Count = 1 Then
If Target.Column = 3 Then
If Target.Row < 6 Then
If Val(Me.[c6]) > 1 Then
MsgBox "Greater than 100%"
End If
End If
End If
End If
End Sub

Answer 3

You can use a UDF to replace the SUM function:
Function SumAndAlert(rng As Range) As Variant
SumAndAlert = Application.Sum(rng)
If Application.CountBlank(rng) = 0 And SumAndAlert <> 1 Then MsgBox "not 100%"
End Function
but I think it might be a little annoying to have message boxes poping out while entering data.