Thank you for this explanation. From my point of view, this makes this tradition a bit unpredictable. As far as I can tell, there also seems to be no way to find out how may incoming trade routes you have.No. The trade income strictly refers to the yield you get after receive a trade route from other civs. Anything that says +X on imported resources (such as Qing's ability) are not counted as trade income.
Also, the amount of trade income you have, is this listed anywhere in the game? The only way to assess the impact of this policy seems to be to slot or unslot it and observe the difference in your gold balance.
I think traditons like this suffer from the intransparency of the trade route system.