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.
To convert from a percentage of a frame to an angle of view, use:
where A is the area of the film frame, p is percentage of that frame expressed as a fraction of 1 (e.g. 9% would be 0.09), and f is the focal length of the lens. A's unit must be in square units of the same unit f is expressed in. That is, if f is in millimeters, A must be in square millimeters. (And now you know why I write utility programs rather than solve nasty little equations like these by hand!)