DigThisData calculates your total product sales over a period of time (default is 12 weeks, however you can change account defaults in your Account Settings) and divides it by the number of days (number of weeks x 7 days). That will give us your average daily sales which the system ROUNDS UP. If your daily average sales are 9.2, DigThisData will display it as 10. This is because sometimes daily average sales can be 0.2 which would round down to 0, and calculations would then render all zeros.
The system then calculates how many days, using your daily average sales until the product reaches either Sold Out (zero inventory) or the Low Inventory Threshold for that location (set in Data → Configure Stores).
Current Stock / Daily Average Sales = Stock Out (for Sold Out as target)
(Current Stock - Low Inventory Threshold) / Daily Average Sales = Stock Out (for Low Inventory Threshold as target)