This site holds the software and support documentation for SACALC5, an app for Windows (64 bit) that calculates the average solid angle. You can download the software without directly contacting the author. The papers that give detail on the software development are published in Radiation Protection Dosimetry and Nuclear Instruments and Methods.
SACALC5 is a program for a Windows computer that calculates the average solid angle subtended by various detector shapes: cylindrical, well, annular window, cuboid, spherical and ellipsoidal, including window detectors, to sources shapes: point, line, disc, annulus, cylinder, cylindrical tube, rectangle, cuboid, rectangular tube, surface sphere, solid sphere, ellipse and ellipsoid, at any position and orientation to the detector. The program also calculates the number of hits on the detector sides and the average path length through the detector volume (assuming no scattering or absorption). The software takes advantage of developments in pseudo random number generators, parallel processing and 64 bit computer architecture to achieve accuracies of 0.01%, and feasibly 0.001% in reasonable time with higher core number workstations.