MOS Amplifier Circuit - Lower 3dB Frequency Calculation
Yes, you need to know the output node (drain). It makes no sense to have it at the source because there is no low-frequency cut-off point because the source voltage largely follows the gate voltage.

The low frequency cut-off (3 dB point) with respect to the drain as output is when XCs = Rs. At this point the gain reduces down to a level dictated by Rd/Rs and the gain remains at this level down to DC.