Inventory Accuracy

I am working on project in a manufacturing company that is focusing on ensuring proper inventory accuracy. One of the main challenges occurs when items are returned to inventory after a part of the inventory is used to assemble the finished product. Return counts are inaccurate which leads to either buying more of the item unnecessarily, or running out of an item during an assembly run since the system says there is more of the item available than there is.  2 cost savings that could be measured are       1. Reduction in buying items unnecessarily – hard to measure since we’re not sure how often this happens in a very large warehouse. and 2. costs associated with setting up to assemble a product and running out of an item and having to start over.  Does anyone have any suggestions about how to capture hard savings in an inventory accuracy project?