Try to match windows desktop to 90 degrees turn in game.
Ex: 1920x1080@800 dpi, 1920/800=2.4 inches. 2.4 * (360/90) = 9.6 inches -> 360 distance in calculator -> 7.1 ingame.
It's a statring point, find your best at +/- 10% range (in inches), i.e from 8.64 to 10.56 inches.