o Calculation Formula – Once you know the pack size used in the calculation you can do the math to determine if the extension cost is correct. Common calculation errors occur with items which are sometimes purchased by the ‘case’ and sometimes purchased by the ‘each’. Some programs with take the last purchased price and consider that to be the ‘case’ price even if it is an ‘each’ price. So, in our example with the black pepper, the formula in the system would be “3 each 5 pound containers per case”.