How do I calculate the DC blocking capacitor at the output of a transistor amplifier?
The capacitor forms a high pass filter with whatever impedance is in series with it.  That series impedance is the impedance of the output stage plus the input impedance of whatever is being driven.

Since the cap causes a high pass filter, the filter rolloff frequency needs to be at or a bit below the lowest frequency of interest.  The full "HiFi" audio range is considered to be 20 Hz to 20 kHz.  If you care about the full range, then use a value a bit below 20 Hz, like 10-15 Hz.

Once you have the frequency and the impedance the cap is working against, use the formula you quote to find the minimum capacitance.