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I have mentioned this before, but one of the easiest method is to draw a point exactly 5 feet behind your sweet spot. Then angle the speakers towards that point, the two speakers forming two points, and the draw point a third point of a triangle. You can use imaginary lines, or use a helping hand to hold a long tight rope.
you have to make sure you start to measure from the mid point of where your TV/screen is and mark the spot 3 or 5 ft behind you as Venkat noted (place a tennis ball at the point) Then do what Venkat told you. YOu can also use a removable tape to measure the triangle