Actually, you want to use MAX() rather than SUM(), as sum will not always equal the value you set. Possibly due to some kind of aggregation that is occurring.

Yes, that's a much better workaround, thanks. Though still seems unnecessary.

This would also allow you to use a calculation to store a single value to use for conditional formatting, which doesn't work now because attributes don't function properly in a condition such as < or > etc. This would make updating format limits much easier to maintain.