Bonello is a standard, but we look at the distribution of axial, tangential and oblique for the rectangular dimensions, as well as deviations and density graphs that are proprietary. There is some very easy to use software from RPG (Room Sizer, Room Optimizer) that are also quite helpful once you spend some time with them. I've had amazingly accurate results from existing rooms plugged in Optimizer...

Trouble with most of these techniques is they don't allow for absorption beyond the possibility of inputting Absorption Coefficients for overall surfaces..

Of course, as Thomas points out with complex geometry it becomes a whole new ball game and experience has to rule. This is one of the main reasons we preach about rectangular rooms for DIY projects. It keeps the simulations manageable and at least you have a reasonable idea of what to expect.