The roughly 1/3 is a pretty good rule of thumb, as is 3 ft from side walls. The system I have used for years winds up right around there. Remove spikes, put speakers on cardboard, towel, carpet, anything to make them easy to move around. Then do move them around, farther out, farther back, farther apart, closer together. For every place you put the speakers also listen from a couple different locations closer or farther away. Listen only for tone or frequency response, or your overall impression of balance.
If you want to try unusual stuff like diagonal placement this is the time. Now they are very close to where you will want them you can put them on your Podiums or whatever. Toe them in about right, and measure to get them symmetrical. Now listen, this time for imaging. Experiment with toe until you get the balance of image depth, focus, and width that you want.
Finally, now that you know where the speakers and chair will be, you can work on the room. I would not even think of doing acoustic panels, tube traps, or anything like that until after first having put the speakers on Podiums and with as many HFT kits as you can manage. Because otherwise you are trying to correct what seems to be room acoustics but are really resonance problems the speakers directly vibrating the room by not being isolated. Huge difference.
