In Andy Pruitts book he put this question in the "what we dont know but wish we did" section. It makes a huge difference for me and each foot is different. Trial and error and feel have been my guide.
The "correct"Q is also closely linked with the amount of arch support and varus/valgus wedge you use. Also adjusting Q will likely change the required angle on your cleat. I have messed about with this extensively and it can make a huge huge difference to foot and knee comfort