Look at your historical usage
Pick a date range and evaluate how much of a product you sold during that time period. For example, you used 500 bottles of whiskey over the last quarter (12 weeks).
Determine your average usage
Divide the total bottles used in a period of time over the number of weeks to see how many bottles you will use in a week. For example, if you divide 500 bottles by 12 weeks your weekly usage is 42 bottles.
Set your Par
Now that we know the weekly usage we can determine the par levels based on your order frequency. If you order weekly, it is recommended to keep 1.5 your weekly usage as par. For example, if your weekly usage is 42 bottles, you would multiply that by 1.5 to get 63 bottles. This is now your Par level and your ordering should be based around it.
Order for your needs
The goal is to have your par remain constant, however life happens; maybe a deal is too good to pass up, or an event is on the horizon requiring a larger than normal order, in either instance you may order more than your Par. When this happens your Par should remain the same, but your on hand inventory will change. You now will know what products you need to order versus those that you may have in excess.