Actually the basis for finding the 35mm format focal length is to calculate the focal length on 35mm format that would give the same angle of view as on the prosumer digicam for the given actual focal length of the lens on the prosumer camera.
The angle of view is a function of the sensor size as well as the focal length.
A simplified model is to imagine an isosceles triangle with the base equal to the diagonal length of the sensor and the height being the focal length of the lens. The angle of view is thus the angle of the tip of the triangle.
Angle of view = 2 ATAN (Diagonal/[2 * focal length])
So the equivalent focal length at 35mm format can be expressed as:
Focal length (35mm equiv) = Actual focal length * [Diagonal of 35mm film / Diagonal of CCD]
Since not all prosumer cameras use the same sized CCDs, so there isn't a single multiplier you can apply to any camera's focal length to find it's 35mm equivalent. Having said that there aren't really that many differenct sizes, so 4.5 seems to work in many cases.
For my Nikon CP995, the actual focal length is 8-32mm, and the 35mm equivalent is 38~150, so the multiplier is more closer to 4.7.
- Roy