...for some styles that is.
Thinking about it just now. 19L of wit, gassed up to 3 vols, contains 19L x 3 or 57L of CO2. That will dispense itself from the corny without any problem, and will release CO2 while doing so, which is trapped in the corny, maintaining pressure.
So, the question is, with a wit of say 3 vols, when the keg is half empty what is the carbonation level of the beer at that carbonation level? The headspace in the keg will be approx 9.5L but as it's under pressure this is more than a half of 1 vol.
Hmm, this is a bit head melting.
You also have to factor in that no system is 100% correct. There is no way to figure (on a hb level) if your system is doing what it says it is...ie dissolving the CO2 your reg tells you it is dispensing. It is a bloody complicated issue when you start to delve deeper but do you need to? It could mean you have lots of lengths of beer line to suit carbonation levels and multiple regs... check these out
http://www.micromatic.com/direct-draw-draft-beer-system-aid-121.html
http://byo.com/stories/techniques/article/indices/39-kegging/165-balancing-your-draft-system-advanced-brewing
http://www.kegking.com.au/balancing%20your%20keg%20system.html
TBH I kinda gave up and just learned by trial and error (with the knowledge I had amassed) and it works out ok but the high carbed beers can be an issue at times.