Don't overlook sales when reviewing your food cost results. There are several important reasons to scrutinize the divisor in the formula as much as the net purchases total. Your sales figure depends on covers, check average, promotions, coupons, discounts and lost revenue.
Lost revenue is related to menu items your service staff can serve a guest with no order entered in your POS system. Typical examples include slices of cake or pie, small pastries, coffee, tea, cocoa, soup, rolls and ice cream. Less common items include modifiers normally ordered through the POS system which have a small charge to the guest.
A second source of lost revenue involves unauthorized voids and price adjustments. I remember working with a family restaurant with a special price for ice cream sundaes for dinner patrons. Anyone visiting the restaurant for dinner was offered a sundae for 99 cents. After reviewing several POS data tables, I noticed late shift changes on many $3.99 items to $0.99. These patrons did not have dinner. They all paid cash. Their orders were changed from the full charge for the sundae to the dinner special price.
My annual estimate for this activity was $10,000. As you can imagine, these changes had to be made by a manager. The wait staff did not have the authority to amend closed checks. If the managers on shift were pocketing cash, you can be 100% confident the wait staff felt comfortable serving items not ordered through the POS system.
Chronic lost revenue won't show up in check averages. If this theft has been going on for years, the check averages will be consistent.