Suppose I am iterating with Euler's forward method on finite differences like so
$$ z_{n+1}(k, l) = \begin{cases} z_n(k, l) + \frac{1}{4}\left(z(k - 1, l) + z(k, l -1) -4z(k, l) + z(k, l+1) + z(k + 1, l)\right),& (k, l)\in\Omega \\ b(k, l),& (k, l)\in\partial\Omega \end{cases} $$
Then, $z_n(k, l)$ will converge to the solution of laplace equation in $\Omega$ with boundary values given by $b(k, l)$. How can I estimate the relative error in $z_n(k, l)$, given the range of $b(k, l)$, and the area of $\Omega$.