"Be Virtuous as Water" is an excellent Tenet that grants a city +2 Stability on river tiles that are either in the city's exploitation or have districts placed on them.  The actual behaviour of this tenet on city stability seems to work just fine:

- A river in the city's exploitation will contribute to that city's "+X from exploitation" count

- A river with a district built over it will cause that district to lose less stability, e.g, a Farmer's Quarter built on a river tile will contribute (-8 Stability from Farmers Quarters) instead of -10.

However, the "New District Predictor" math is not correct because it treats the new district being built on a river tile as losing the +2 bonus, and doesn't include new rivers in the exploitation, which often shows new river districts as -12 Stability.  The correct total (in the case of the screenshot below) would actually be -8 because we're building a new Farmers Quarter (-10) but also gaining a new river tile into our city's total area (+2).

To fix this, the "New District" prediction should include the following factors if the "Virtuous as Water" tenet is active:

- Include +2 stability for all rivers in the new exploitation

- Do not subtract -2 stability for building a district on a river because the bonus stability will still affect the new district