The focal length of a lens or telescope is independent of the size of the sensor of the camera used. So 200 mm remains 200 mm regardless of if you use a 1 x 1 mm sensor or a 100 x 100 mm sensor. Ekos can indeed compute the new FOV but you need to clear the DSLR info in the imaging tab before you start imaging. There is a trashcan icon that you need to click and then Ekos will ask you for the new info.