Balancing RCS between COM and DCOM?

[guitarxe]

So if you use RCS Build Aid you can see the COM as well as the DCOM of your craft in order to balance RCS placement, which brings me to the question... which one do I use?

Currently the craft I'm trying to balance is a SSTO spaceplane, so it will have used up most of its fuel upon launch, so I should obviously use the DCOM for balancing, but it won't be completely out of fuel by the time I have to dock, so I can't really get this perfectly balanced anyway.

And once I'm docked to my station and refueled, if I want to re-dock again for whatever reason or dock at another station, then I would have full or at least half-full tanks for that, so again the balance would be off.

So then how should I balance the craft? Would it make sense to keep two sets of RCS thrusters, one for empty and one for full tanks?

[AlexinTokyo]

My way of doing this (for rockets, I'm not much of a spaceplane person):

1. Display the ACoM (Average CoM) using RCS Build Aid.
   
2. Place the RCS thrusters in order to minimise translation torque at ACoM.
   
3. Check the maximum RCS torque at CoM and DCoM.
   
4. Ensure that the vessel has sufficient non-RCS torque (i.e. reaction wheels, command pods) to overcome this max RCS torque.
   

[Reddeyfish]

the COM and DCOM are close enough that this doesn't matter. The unwanted torque created by the slight imbalance can just be canceled with reaction wheels.