One method for calculating $Z_{\text{eff }}$ is to use the equation:

$Z_{\mathrm{eff}}=\sqrt{\frac{\left(E\right)\left(n^2\right)}{1312\mathrm{kJ}/\mathrm{mole}}}$

Where E is the energy necessary to remove an electron from an atom and n is the principal quantum number of the electron. Use this equation to calculate $Z_{\text{eff }}$ a value for the highest energetic electron in potassium. $E=418.8\mathrm{kJ}/\mathrm{mole}$