There's no perfect source. Of course taking one set of numbers can mislead.
But there's enough datapoints you can use to get a very good idea. If NPD and VGChartz and your own numbers over the years (the only ones you can 100% trust), plus accounts from people inside the industry at every level (retail, publishing, etc), and public charts and steamspy and (etc etc etc) all show very similar patterns between similar games then you can be pretty confident of the ballpark you're playing in...
But that's my appeal to authority... People are sick of "experts".