I'm not a lua coder, and it seems the algorithm is hardcoded so I can't find it in the lua files of SupCom, so the only thing I can provide is a speculation.
5km maps have 20 zoom levels, get 45°rotation at level 10.
10km maps have 23 levels, and the last one isn't a full one, so it's actually 22.5.
20km maps have 25 levels.
81km maps have 30 levels, get 45°rotation at level 15.
Also if you try to zoom in to the north or south edge of a map, it feels like the trajectory of the camera movement come close to 4/1 of a circle.
So it seems the algorithm is really simple, the rotation angle gradually decrease from 90° to 0° at a constant rate from fully zoomed out to the ground. For example, on a 81km map, which has 30 zoom levels, the camera rotate 3° for each zoom level.
Other than this rotation and how SupCom limit the camera movements (force it to be contained in a camera shaped space which also contained the map), the SupCom camera is COFC without free rotating.