This calculator can be used to calculate the Field Of View of both fisheye and normal lenses and the number of images needed for a panorama using a given overlap. You can also find out how much overlap you get with a given number of images. By playing with the number you can optimize your shooting strategy.
The calculation for fisheye lenses is theoretical. If the image is larger than the image circle of the lens (i.e. circular fisheye) the calculator will not warn you for the fact that the image area will not be covered by the image circle of the lens.