Calculating months of inventory on hand

I receive customer's sales by units, dollars and on-hand units each month.  I copy / paste this into a master sheet with month / year as the date range dimension.  I'm trying to calculate Months of Inventory on hand by using the following method:  Units on hand / Unit Sales.  I have Unit Sales as a running average so it's taking the totality of the sales and averaging it out by month.  I'm only using the last period of time where inventory was reported so there is not a full sum of inventory.  My calculations work for some items but are way off for others.  Has anyone been successful with this?  

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