In trying to understand the Nernst and GHK equations, I've hit upon a snag somewhere deep in my understanding of the subject matter.

Scenario 1: when calculating the membrane potential of a living cell at rest we use the Goldman equation. This requires us to measure both the concentrations of the ions and the membrane's permeability to said ions, at rest (at least for the concentrations).

Scenario 2: given two cells separated by a membrane (as in Figure 3 here), with a different concentration of KCl in each cell, and having the membrane permeable only to K ions, we can compute the equilibrium membrane potential using the concentrations of the K ions - after the system has reached equilibrium.

So far so good. Here's my question:

Given a living cell at rest with all its ion concentrations, the Nernst equation is used to compute the equilibrium potential of a single ion, using the concentrations at rest. Why is this allowed? As I understand it, the concentrations as measured are what the cell "decides" them to be - it can change the permeabilities of the channels, or create more pumps, etc. - and then we'll measure different ion concentrations at rest. And these concentrations will change if a cell decides to open or close some channels (which is what happens to create the action potential). But the concentrations that are used as input for Nernst's equations should be measured at the actual, not just the steady state, equilibrium (right?).

So, why can we use the steady state measurements to derive the equilibrium potentials?