I had the delightful opportunity to go to a Masterclass with Sir James Galway (one of my heros!) and here is what he said: Place your weight on you right foot, lean back slightly onto that leg, and tilt slightly forward at the hip. Tilt your head a bit to the right, but keep the flute at a right angle to your lip. By doing this you have a strong stance, and can move a bit to enhance your performance. The right arm gets a bit of a rest when you are playing for long periods of time.
I tried this stance..but here is the wild thing: I found that FOR ME, I have much better tone when my weight is forward on my left foot, my head erect, lifting from the back of the head, and flute parallel to the floor. (It does make the right arm work more.)
Try placing your weight in various places--left foot, right foot, central on both feet. Try a slight lean from the hip,or a tilt of the head (always maintain flute at 90 degrees to lips). Experiment and explore until you find the right posture to give you beautiful, round tones.