I have two suggestions.
One, call a company called Bevinco at 1-888-Bevinco or
www.bevinco.com . They are an inventory control company and they will weigh your kegs and match up your usage to sales. As the moderator said, this is the only fool-proof way to get a handle on this stuff.
Second, there is a company out of Wisconsin that sells something that might help. Check out
www.turbotap.com
Ian