Need Equation for Hiqh Pass Filter capacitor

In the process of building a preamp, I am wanting to install a high pass filter upstream of one set of RCA outputs to go to the amp powering the main speakers. The other set of outputs will go to the sub, which has its own Xover. I want to mount a capacitor in the chassis to create the filter. I'm looking for a correct equation that relates the reduction (in dB) to the frequency and the capacitance. That is, relates the variables:

dB offset
frequency
capacitance
input impedence of amplifier

With this, I will plot out a curve and determine the corner frequency of the high pass filter. Can anyone offer some steerage on this one?

Thanks Alot, Peter