As with most placement type issues, once you've started somewhere around the "correct" position, it's really just time to experiment. Grab a movie that you feel has good surround effects, find a particularly good scene. Move the speakers to a different position and see where you think you had the best effects. If you have these wall mounted instead of stand mounted it will be a little bit of work but you can use anything as a temporary stand, using two friends usually works out well.