By default, when computing sky lighting, Cube 2 casts 17 rays at a spread of 37 degrees from a vector pointing upward. This patch adds two commands:

skylightrays - controls the number of rays (default 17)
skylightspread - controls the angle of the spread of the rays (default 37, goes up to 180)

The mod comes as a patch that you can apply to the sauerbraten source code, you'll have to recompile it yourself.

Please note that when you calclight, you will lose binary compatibility with older versions of your map, even though visually it should look very similar if you use the default values.