Conversion/overhead cost allocation

Hello,

Currently, we are utilizing the average cost method for our inventory. However, we encounter a challenge when it comes to incorporating conversion/overhead costs (such as labor, utilities, services, etc.) into our inventory items. These costs are only known once the build is completed and we receive the corresponding invoices or data. Is there a way to retrospectively include these costs in our inventory items?

Since we don't have the exact amounts before receiving the invoices, it is not feasible to include them in the initial Bill of Materials (BOM). Additionally, the conversion/overhead costs vary on a monthly basis, requiring changes to the BOM. While we incur the conversion costs on a monthly schedule, our assemblies take place throughout the month. Therefore, ideally, we would like to allocate these costs to all assemblies completed within a given month.