# How do I determine the accuracy of this two-dimensional potential field?

Let's say I want to solve this problem. I know the values on the boundaries and I guess an initial solution on the rectangular grid inside these boundaries, see figure below. Potential=10 on boundaries; potential=0 inside

I know that eventually the field will converge to the same potential as the boundaries. If I let the program iterate long enough the whole area on the inside will also become yellow. The figure below shows an intermediate step towards equilibrium:

Now, in this project I am working on I am supposed to stop the simulation when the accuracy of the simulation is 1%. Is there a general definition of accuracy in these cases when working with a two dimensional grid? There are several grid nodes, all with different values, are these supposed to be 1% or less from equilibrium (all yellow)?