For purposes of calculating deviation, you can take the prism to be a right triangular prism. Since the plane of incidence is specified to be parallel to the bases of the prism, the problem is essentially 2D: the incident ray gets refracted by two adjacent edges of a "triangle", which define the apex angle $\alpha$. The deviation angle $\delta$ simply refers to the angle by which the outgoing ray differs in direction relative to the incident ray. It will depend only on the angle of incidence, $\alpha$ and the refractive index of the prism.

The following figure from Wikipedia illustrates the geometry. The deviation angle is $\theta_1 + \theta_2$, and the article also gives an expression for $\delta$ although it sounds like you are not looking for that.

For purposes of calculating deviation, you can take the prism to be a right triangular prism. Since the plane of incidence is specified to be parallel to the bases of the prism, the problem is essentially 2D: the incident ray gets refracted by two adjacent edges of a "triangle", which define the apex angle $\alpha$. The deviation angle $\delta$ simply refers to the angle by which the outgoing ray differs in direction relative to the incident ray. It will depend only on the angle of incidence, $\alpha$ and the refractive index of the prism.

The following figure from Wikipedia illustrates the geometry. The deviation angle is $\theta_0 + \theta_2$, and the article also gives an expression for $\delta$ although it sounds like you are not looking for that.