It's useful to think about how skirts work. We have three types commonly in use:
1 Lift air fed via plenum no divider - often segmented type
2 Lift air fed by plenum with divider - often segmented type with "Bryan White" divider
3 Lift air direct fed to rear compartment with divider. Often Sev type.
Other configurations are possible!
For type 1, any pressure above cushion pressure in the plenum is waste energy. The job of the designer is to reduce theses losses by making all the passageways and holes a large as possible. So I would say less than 1 mBar is desirable, as low as possible to reduce losses.
For type 2, when the bow pitches down, reduced air leakage under the skirt must cause increased pressure in the bow cushion compartment. This can only happen if the plenum is at higher pressure, so it must therefore be running at a pressure some amount above the cushion. The exact amount needs to be determined by the designer and will depend how much pitch up moment is desirable. So perhaps 1mBar is enough, but more is better - except that this is a loss of lift power so you can't go too far.
For type 3 the bow compartment main air feed is under the divider, and this compartment is designed to run at cushion pressure or less. In normal flight the now compartment runs at a pressure below the main cushion pressure - in the region of half to two thirds. When it pitches down, fron curtain seals but the divider is still open and the pressure rises quickly to full cushion providing the required pitch up moment.
From these basics it's possible to work out the maths. Type 3 is the most efficient, type 2 is the least efficient but can right a plough in if well designed and type 1 has no ability to right a plough in and is less efficient than type 1.
Incidentally, Kingfisher was a rare beast in that it had a rear fed divider skirt in a segment craft. It worked well, and is the way forward for segment skirts, but the craft must be designed for it at the outset.
Ian