SQL Server 2012 :: Calculating Inventory Turns

So I have been struggling with a way to perform inventory turn calculations in SQL. I'll start off with the formula. Basically the formula is Cost of Goods Sold over last 12 months / Average Inventory Value over the last 12 months. Short hand as COGS/AvgInventoryValue = Turns

In order to obtain the COGS value I take the Avg(UnitCost) * Sum(InvoicedQty) in the last 12 Months for Sales documents. This is represented as DOCUMENTTYPE = 2
Then divide AvgInventoryValue
To obtain the AvgInventoryValue I need to know the Quantity purchased in the 12 month period * Avg(UnitCost) + QuantityOnHand at the start of the period * Avg(UnitCost)
The purchased documents are represented with DOCUMENTTYPE = 6

The purchases will be positive since we are putting value in. The sales are negative since value is going out. I'll start with a simple example where the product is at zero Quantity On Hand at the beginning of the period.

--CREAT THE TEMP TABLE
create table #VALUEENTRY
(
ITEMNO VARCHAR (50) NOT NULL
, POSTINGDATEDATETIME NOT NULL
, DOCUMENTTYPEINT NOT NULL

[code]....

Is there another approach I should take to obtain the inventory turns?Also how do I account for periods earlier than 12 months where inventory is left over?In the sample data this item happened to be at zero inventory on hand at the start of the period. Also I believe it is my subquery's which are throwing the numbers off but I'm not sure how to isolate the subquery to just perform the result for a specific item. It is aggregating the entire table. How can I make the subquery more specific?