Often pressurise part filled barrels, as like to split 23l batch between any any empty lever tops and remainder in King Keg. Makes Barrel a bit lighter to carry out to garage, and a few bottles for visiting. The other big advantage is that with the bigger initial headspace, pressure from priming lasts to disspense much more before repressurising. With a 12.5l fill it lasts the whole barrel.
Two things you might be considering. Getting carbonation pressure right, and purging to prevent oxidation from Oxygen in headspace.
PURGING:
I don't worry too much residual Oxygen being absorbed. If there's yeast left in suspension, particularly hefeweizens "wheat beer with yeast", the yeast helps stabilise the beer.
But to purge most of any remaining air, calculation for CO2 weight needed:
At atmospheric pressure of 1.01 bar, 1.8g fills 1 litre, ie 1.8g/l
Sparklets are 8g (though I use S30 CO2 cylinders), so 1 sparklet gives 4.44 litre of CO2
King Keg is 27l to brim. So after:
23l fill, add 4l CO21 (1 sparklet)
19l fill, add 8l CO21 (2 sparklet)
15l fill add 12l CO21 (3 sparklet)
Do with lid only on loosely, so air can be purged. There's some mixing, but there's much less Oxygen remaining.
CARBONATION PRESSURE:
King Keg is rated 10-12PSI (0.69 - 0.82 atm.)
At 20C and 1atm. pressure, there's 1.836kg CO2 /m3, or 1.8g / litre.
King Keg instructions say use 99g priming sugar (for 19l fill) which is 4.3g /l.
99g of fermented sugar produces 43.5g CO2, that's 24.1l at 1atm.
Some of this CO2 is absorbed into solution.
The amount of CO2 dissolved in water is proportional to pressure. At 20C, 1 liter water dissolves about 1.7 g CO2 at pressure (1 atm), 23l water would hold 39.1g.
At KingKeg pressure (0.82 atm) an aditional 0.82 x 39.1g = 32g would be dissolved (in 23l).
Assuming as much CO2 dissolves into beer as into water, that leaves 43.5 - 39.1 = 4.4g CO2.
4.44g CO2 occupies 2.44l at 1atm. 20C
A 23l fill, leaves 4l headspace, so additional pressure is 2.44/4 = 0.61atm (9 PSI).
So with a half filled barrel (12.5l fill) headspace is 27 - 12.5 =14.5l
Dissolved CO2 = 32g/2 = 16g
For pressurising headspace to 0.61atm. CO2 = 14.5l x 1.8 g/l x 0.61 = 16.g
So total 32g CO2 required. That needs 73g priming sugar (which is about 6g /l).
I put all the beer in the barrel; stir in dissolved priming sugar; purge if if I think necessary; close lid; pressurise just enough to dispense, then fill bottles from fill tube on barrel tap.
So when botteling half the batch, I prime full batch of 23l with 138g (6g/l), which is close to the normal 1tsp / 500ml bottle rate, then bottle half from barrel.
I'm typing calculations in from my envelope notes, please watch for errors.
I have S30 connectors on my all barrels. S30 CO2 cylinder holds 240g, refill/exchange under £8 locally, and you can put in short or long burst as required.
Keep 2 cylinders, one's usually full. Occasionally you find the last quarter or so has emptied emptied itself, guess as pressure drops cylinder valve is not fully closing. But you're warned not to put in very long bursts, as valve can freeze open.
Have just replaced oldest brass one (30 year old) with new stainless type (brass tarnishes, but does polish up fine). Looks neater, but it's harder to check the seal rubber sleeving.
All barells are top tap, so can still get glass under if on floor, with floats - means you're drawing ber off just below surface, so becomes clear much faster (after priming).
Recently upgraded to rotating floats (with silicone pipes). Old ones occasionally started dispensing mostly gas as level dropped, but new type good so far.
Deluxe sparkler tap upgrade on two old barrels. Like lever action, but tend to leave sparkler nozzles off (says to wash after use).