I've gone through this post in Dev forum, am not sure if its too late to suggest a modification to stock_move table.
There should be a costed_date column, to show the actual date/sysdate and time the transaction was done, which is a different value from the transaction_date column (mostly because of backdated transactions)
sorting transactions by this column will help anyone to know the exact movement of average cost.

Thanks for the good work you're doing.

Regards.
Mugao.