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To calculate the actual field of view of a telescope you need to know the actual focal length of the scope and the field stop of the eyepiece.
Then the formula is; field stop/focal length times 57.3.
So for your example you might go something like this;
40.8/2000*57.3 = 1.16 degrees.
However when you put a 2 inch diagonal on a SCT you effectively make the focal length longer. The nominal focal length of 2000mm is when fitted with a 1.25 inch prism diagonal. The SCT focuses by moving the primary mirror closer to or further away from the secondary mirror. From what I have read an 8" SCT with a 2 " diagonal is more like 2200mm focal length.
So our calculation is now;
40.8/2200* 57.3 = 1.06 degrees.
Still doesn't agree with your findings but you can see there is a bit more to it than you might think.
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