[petek]

Calculate the area of a circle of the larger diameter (L).
Calculate the area of a circle of the smaller diameter (S).

The area of the total ring (R) is R = L -- S.

Take the area R and divide it by 360 degrees or 2 PI radians (or even 3600 degrees or 20 PI radians) to get the area per degree or tenth of a degree.

Measure the opening as number of degrees or radians of a circle. Multiply that figure by the area per degree.

Sound right?