Ice, aces high models them after historic tankage values. You can find them online with google or your search engine of choice. Also, it has been discussed on this forum and even though our search doesn't work, these forum threads still turn up on google as well. It may give you the results you want.
Also, you described perfectly how to determine fuel in each tank with E6B. Turn the fuel burn up and sit on the runway. Manual fuel select each tank until it's dry. Take down the numbers, do the math, and there you have it.