Hi Mike
If you keep transaction history (and everybody should because there is no downside to not doing it), then the utility Toolkit for Adagio by Dakota Software (
www.dakotasoftware.com) can retroactively compute the correct sales figures for you, with press of a proverbial button.
Yes, I realize that Toolkit for Adagio costs money, but your effort is worth something, and there is also a greater chance that you will do something wrong the way you are trying to do it.
If you already own Toolkit for Adagio, it's a slam dunk.
Steve