And every year, she recalculated the indices using the latest three years of data, because seasons shift. A new boardwalk hotel opened, boosting spring sales. Her Spring Index crept up from 0.99 to 1.10.

"Yes," Leo smiled. "An index of 1.0 means 'exactly average.' Below 1.0 is low season. Above 1.0 is high season." "Now you can predict next year," Leo said. "First, forecast your total sales for next year using a simple trend—say, you expect 10% growth because you're adding outdoor seating."

He drew four boxes on the napkin. "First," Leo said, "write down your total sales for each season for the last two years."

She implemented the system. The following summer, she ordered 80 gallons of chocolate fudge instead of 40, and she didn't run out once. In winter, she launched a small hot cocoa and cookie menu (index 0.34 meant low volume, so she kept it simple). She stopped wasting money on full staff in January.

"You're fighting the seasons blindfolded," Leo said, sipping a lukewarm coffee.

"Now," Leo said, "calculate the across all your years."