In general what should I try to do?
* Check off right values: First, try to find cells whose values are right and place a checkmark. (This is how you test.)
* X out wrong values: If you find wrong values, place an X mark. Making decisions (placing checks or Xs) on the cells and the interior cell colors help to narrow your search for the error.
* Fix the formula errors: Using the yellow/orange interior colors to find the erroneous formulas, carefully fix formula errors you find and then retest the values that result.
* Test thoroughly: Change values to cover situations that have not yet been tested. This will help you make progress in testing.