Explanation for what this calculates here. In short, this page calculates, from a given Blender camera angle for an orthographic or isometric camera, how the angles and proportions of x,y,z sides will come out in a 2d image.

Blender Camera Angles:

Insert values between 0 and 90. Normal values for X and Z, for a perfect isometric projection, are 54.736° and 45°.

X°
Z°