Alternatively, you can solve the "optional" decimal point problem using the following solution:
Number Format: General;[Red](General)
This will add the decimal place and fractional value accordingly, while formatting negative numbers in a more legible way.
As for the poster's original question, you still need to round/truncate the "extra" decimal points using a formula. However, this is a simple formula of =ROUND(<value>,<desired decimal places>) that is not extremely computationally expensive.
Examples:
2500 -> 2500
0.25 -> 0.25
-2500 -> (2500)
-0.25 -> (0.25)
