In my current game, spending is 100%, tax rate is 50%, morale is 100% and my surplus is 600bc. How large can my treasury be before it affects my score?
I'm assuming you're worried about an Anomalous score. I can tell you that the treasury flag is only tripped by getting a lot of money in the early game so my suggestion is to keep your treasury around 5000 to 6000 BCs early on. Any more than that and you will risk going Anomalous.