How much headspace to leave when fermenting in a corny?

[Frisp]

As the title asks How much headspace is needed to pressure ferment in a corny? Or a unitank for that matter.

is there a magis ratio?

 

[AJA]

As little as you dare... I cut of the gas post to almost flush, and leave a litre. This isn't always enough, even at 10psi and I sometimes get some krausen through the sounding valve.
So 2l to be safe id say