Naturally carbing a half full keg

Is it possible, or is that way too much headspace? I'm splitting a batch, bottling half and kegging the other half. The whole batch will be primed. I'm planning on hitting the keg with 30-40 psi and leaving it be.

I don't know what I'm asking, I guess I don't really know how much gas is given off when a batch is primed and how much gas is absorbed into the beer.

See anything wrong with this?

Oh cool, this is the 11,000th thread.

Purge it once and then re-pressurize to 15 psi. That with the priming sugar should get you into the ballpark.

David's advice is good. fill the headspace with CO2 at a psi that's close to where you'd set it to force carb...then the headspace doesn't figure into the priming sugar needs, and just add what would be needed for 2.5 gallons of beer. it might be slightly over carb'd but that's an easy thing to fix by venting once or twice when its time to dispense. i'd test it 24 hours before you plan to serve it to guests to give you that 'tweaking' time.