You're not far at all. It's all likely, but not mucho of it really is verifiable. This number is a guess, you could never really know which address owners have lost their keys and which just haven't touched their stack in years. You can make informed guesses by statistics of trashed computers, by following reports on lost keys and some such thing, but even there, who knows if it's not a "boating accident" before a divorce. It's informed guesswork.