batch priming

[newToBrew]

so - I'm due a kegging - bottling evening some time next week,
have 2 PA's ready to go - cascade Amirillo and Cascade RAkue
and a small demi john blend of both
@ 20 - 22 litres of each I think
anyway - have 2 kegs - dual reg

I want to bottle some and keg maybe @ say 12 - 15 litres of each - would i be better just force carbing than priming this given the amount of headspace in the kegs - I'm guessing anywhere between 3 and 7 litres of headspace
for some reason - I'm thinking that this space would have an impact on the priming calculations of sugar amount

does this make sense ? -  I guess i'm thinkin that the gap in the keg would require extra co2 to fill it before the liquid starts absorbing it
again - does this make sense ? - not quiet sure I am describing it right   

[Ciderhead]

co2 will sit above your beer quite a few squirts to clear the air and once the tank is cold and inder pressure co2 will be absorbed.
I think you need a beergun or make Wills patented device

[newToBrew]

i'll probably split it out - kegging batch and bottling batch - prime teh bottle batches and force carb the keg batches