TopSwitch-HX calculating the Y capacitor

Design Team, I have read through the HX datasheet and application note AN-43 for any reference to proper sizing for the "Y" capacitor. I see refernce designs that use 1nF or 2.2nF at 250VAC. My design is a universal input 20W continous design. I would like to use a 1nF 250VAC cap, but will switch to a alternate part if justified. Is there any calculations or "rule of thumb" to follow when selecting the "Y" cap value? Thanks Dan Szubra