OK, but I just think a Math class would help. As for the Primitives.getSkyCube suggestion, it could read getSkyCube(Texture front, Texture right, Texture left, Texture back, Texture down) so as to make texturing very easy. A sky cube is something EVERY game uses, so it's not a trivial suggestion in my opinion.
EDIT: Egon, even though you've not been answering, I've been thinking about this for a bit and I think it makes more sense for the sky box to be a FrameBuffer method (FrameBuffer.getSkyBox(Texture front, Texture right, Texture left, Texture back, , Texture up, Texture down)). And maybe its size could be just > or just < Config.farPlane.