Re: parasytic capacitance formula
I agree with Rattus with one addition. If you are at a high enough frequency to worry about the capacitance of a cable, then you are probably in the transmission line realm. In other words, the capacitance is no longer "lumped". It is distributed along the cable with distributed inductance. That gives the cable its characteristic impedance Rattus mentioned.
Also, the cable is probably not a small fraction of the wavelength of the voltage (or current). As a result, the capacitance your voltage sees will depend on how long the transmission line is, and how it's terminated.
For example, a 1/2 wave line terminated in a short circuit might look like an open circuit. A 1/2 wave line terminated in a open circuit might look like a short circuit. (I say might because I'm not sure I remember these right).
Match the source, load and line impedence, and the the line will look resistive.
Steve