How is the stress concentration factor calculated in FEM?

I have written my own finite element code for a linear elastic (plane stress) problem consisting of a plate with a circular hole in the middle, fixed on one side, having a tensile force acting on the other side.

I have obtained displacements and calculated the stresses and strains at the nodes. I want to calculate the stress concentration factor numerically and compare it with theoretical values for the same case.

I understand that the stress concentration factor $$K_{t}$$ is given by

$$K_t = \frac{\sigma_{max}}{\sigma_{nom}}$$

$$\sigma_{max}$$ is very easy to find but I have trouble understanding how to calculate $$\sigma_{nom}$$ for my problem.

Any help would be appreciated!

$$\sigma_{\text{nom}}$$ is the stress in the component without a hole.