Edit: Sorry, my mistake. After a quick flight in SE, it does indeed appear that the FoV is from top to bottom of the screen, not left to right. The formula below still works, but use the monitor height rather than the width. I have edited were appropriate.
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I believe S.E. displays FoV from side to side, so width of the monitor counts, not the height.
You have two right-angle triangles — from the center of your eyes to the middle of the monitor would be the "adjacent" side, from the center of the monitor to one of the side edges either the top or the bottom (or half the width height of your monitor) would be the "opposite" side.
Opposite over adjacent gives you the tangent of the angle between the "adjacent" side and the hypotenuse (which is from your eyes to the edge of the monitor). So ((arctan of half the width height of the monitor divided by the distance from it) times two) will give the angle from your perspective from one side of the the top of the monitor to the other bottom. Set the FoV to that value, and your monitor should be the equivalent of an actual window into the SE universe.
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So in your actual case, 13.5/2 = 6.75
6.75/25 = 0.27
arctan 0.27 = 15.1096
Times 2 = 30.2192 degrees.
I hope I haven't made an embarrassing error in my math.