How can I calculate sodium excretion over time?

[Dunkhan]

I am trying to simulate salt metabolism in a very rough and simple way for a game. It is more complex than I had anticipated, and difficult to get simple base values. If I have rough hydration levels as a value and something approximating serum salinity:

What would be a rough formula for calculating sodium excretion. This has to handle extreme results like acute hyponatremia and hypernatremia.
What is the maximum excretion at which point the body is unable to keep up with consumption?

This information will not be used for any actual medical purpose on any actual organism, inaccuracy is acceptable and even intended to keep the calculations simple.