I don't see how using Gaussian surfaces can help me to calculate flux for example if there is a disk with radius R and center at (0,0,0) and a point charge at random (a,0,b). If I consider an hemisphere around the disk I still can't calculate the flux through it easily, since the electric field won't be perpendicular to the normal of the surface. Is there anyway to use a Gaussian surface to tackle these kind of problems? I mean something like this:
*I'm not asking for full calculations of course, just an abstract notion of how to treat these cases.