Hi Rich,
I can confirm that in the shipping release of Adagio Inventory 9.2A (2017.06.08), the additional cost fields in Item Assembly are working as expected. When a BoM Master item is assembled, the most recent costs of the component items are added up, and a Cost to Prorate value is added to the master item unit cost if used (the total cost to prorate divided by quantity built) and an additional cost is added to the master item unit cost if used (the total additional cost, regardless of how many built).
The Assembly audit list will list both the Prorated and Additional costs for any given assembly transaction, at the top right of the transaction, just above the total cost of the master item, if you want to investigate how the Most Recent cost of the master item was determined. Divide the total master item cost by the quantity built to get the new Most Recent Cost.
If you find a process that doesn't appear to work correctly, I'm sure your Adagio consultant would be happy to assist you.
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Regards,
Softrak Tech Support