The calculator does what it is supposed to, it uses the absolute FOV for the calculation. For any view that isn't in the center of rotation, be it 3rd person or 1st person, it's practically impossible to do an object based monitor distance sensitivity unless the object is far away. You will have a different sensitivity for objects 1, 10, 20, 100 and 200 meters away etc. You also need to know the exact distance the camera is from the center of rotation (which may vary when you're inside buildings etc). Combine all this and there's so many unknown variables that it's not feasible to do anything accurate.