When creating a Lens Flare (Filter > Render > Lens Flare) press-and-hold the Alt (Mac: Option) key while clicking on the image preview to bring up the Precise Flare Center dialog box where you can input the X and Y coordinates for your flare.
You can use the info panel to see the coordinates of a specific spot on your canvas. Simply hover over the canvas, and you will see the X and Y coordinates change as you move through it.
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