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.
If the actual focal length is 5.4 ~ 16.2mm, then the zoom factor is 16.4/5.4 = 3X.
The zoom factor remains the same (3X) after you convert the focal length to 35mm format equivalent.
Also, the aperture f number remains the same. However, that is only applicable to exposure considerations but not DOF considerations. In other words, a 5.4mm f/2.8 lens on a prosumer cam may have the same angle of view and light gathering capabilities of a a 35mm f/2.8 lens on 35mm format, but the DOF is a totally diff story. That is beyond the scope of this discussion.
The 35mm format equivalent cannot be determined without knowledge of the CCD dimensions, but it is most likely a 35~105mm as that is very common among prosumer cams with 3X zoom. Some models do go as wide as 28mm though, so the best is to consult the user's manual or product specs published by the manufacturer. Since the 35mm format equivalent focal length is such a common parameter that consumers look at, most of the time manufacturers will publish that dats so there is seldom the need to do your own calculation.
As Roygoh has elaborated, the actual multiplier/crop factor depends on the size of the CCD. For most prosumer digicams such as the Canon Gx series, Fuji S602..etc the crop factor is approximately 4.xx. However those mini-compact cameras have even smaller CCDs and will hence have a different multiplier.
The multiplier has no effect on the aperture as the f-number is just the ratio of focal length over aperture diameter. As for DOF and circles of confusion.....that's another confusing issue altogether..