Use a current stock count
Divide usable stock by average daily consumption for a simple coverage estimate. An old stock figure produces an old answer.
Allow for lead time and a reserve
A simple reorder point is daily use multiplied by delivery lead time plus reserve days. This is a planning assumption, not a forecast.
Review the exceptions
Busy weekends, closures, incoming orders and rejected stock change the picture. Discuss those exceptions before placing an order based on a calculator.