Calculating final nitrate level after water change

Hello,

I realize this is probably a fairly simple equation, but I'm really
struggling with it. I'm trying to create a formula (perhaps to
eventually work into a classroom lesson) to predict the nitrate
concentration in the aquarium AFTER a partial water change. This is
complicated by the fact that I use natural ocean water, which, at least
in this area, is NOT nitrate free (low, but not free). I realize I
could just ignore this semi-insignificant factor, but the point of this
is to include all inputs.

Any ideas?
Thanks
Ryan Lenz