20° is quite a small angle. If you are also sending light into the prism at a small angle (say ≤ 20°) to the normal to the first surface), then a good approximation to the total angle, $D,$ of deviation is $$D=(n-1)A$$ in which $A$ is the prism angle (the angle between the faces through which the ray enters and leaves). $A$ must be small for the approximation to hold. $n$ is the refractive index of the material of the prism.
For example, consider a prism with $A=20.00°$ and $n=1.500$. For a ray incident on the first surface at 20° 'below' the normal, accurate (I hope) calculations based on Snell's law give $D=10.26°,$ whereas $(n-1)A=10.00°$.