The precision of a hand-held spot meter is generally measured in degrees, but the in-camera spot meters in advanced cameras are usually measured in percentage of the frame. This is because the actual angle of view of the spot meter changes depending on the focal length of the lens you have on the camera. This formula will help you to calculate a metering angle given a frame size, a percentage of that frame, and the focal length of the lens being used.
This calculation is closely related to the Angle of View calculation, but it different enough that it warranted a tab of its own. Conversely, this calculation might be useful for answering other questions, such as those having to do with a camera's autofocus sensors.
The program first calculates the meter's diameter:
Make no mistake, the above function is very much an approximation. The main assumption is that your camera's spot meter is round, which may be close to the truth, but is not guaranteed to be. For example, the meter in my Canon Elan IIe is a rough oval. Anyway, without knowing the details of your camera's design, this is the best guess the program can make.
Having assumed that the meter is round, the function above calculates the area of the frame that the meter takes up (Ap), A being the frame's area in square millimeters and p being the percentage of that, which you entered into f/Calc. It then runs that backwards through the familiar old "pi r squared" formula; that is, we know the area, so now we want to find the circle's radius. Multiplying by 2, we get our ideal round meter's diameter.
We've reduced the problem at this point to a simple angle of view calculation:
Our ideal round meter's diameter goes into the AoV formula as F, and the focal length as f, giving the meter's approximate angle of view.