This quick tip is looking at lead times. And the tip is: check the data. 

You may have in your mind how long a supplier should take to get you your items. They may tell you that it will be 14 days, 21 days. But what you need to do here is check the data. 

You need to check the time in between when your purchase order is created, and when you're actually receiving those items into inventory. That will be your total lead time. You need to check the data and see how long it's actually taking to get these items from your supplier. 

Then going back to your forecast calculations and putting the real data in, and not just the ideal version of your lead time.