How do you calculate the angle for the image of an object entering a pinhole camera?

you have the length of your camera.. the distance from the pinhole to your film or other media. You have the base and height of the triangle.
B/H will give you the tangent of the included angle.

Good luck... have fun..

So if the camera is in the shape of a cube? what would that make the angle?

A little over 53 degrees.

Let's say the film occupies the entire inside of the cube's rear face, with the pinhole centered in the front face.

The angle made by pinhole, center of film, and midpoint of one side of the film is arctan(1/2), about 26.5°, so the total subtended angle across is twice that, 53.1+°. The angle across the diagonal of the film will be 2*arctan(1/√2) = 70.5+°.