I think I'd start by the defining the purpose(s) of the shield. Are you try to
stop signals from getting out or coming in. Then define sources and create an
equivalent circuit as a road map. Use reactive components. Field solver tools
can help you here. Hopefully you won't need a full wave approach and a circuit
model will do. Here some more hints. Don't make anything a *perfect*
short or open. Don't assume zero volts (ground) is anywhere. Assume current
will flow in your shield. Your job is to figure out how.
I have seen one neat application of PCB shielding with regards to Sony surface
mount SRAMS. It seems that an outer shielding layer helps reduce magnetic
coupling to currents under the low profile surface mount chips. The coupling
can flip bits. ech! Where you tie this to some other ground may not be a major
factor since the parallel plate coupling to other planes and bypass capacitors
may be enough to dissipate the hi freq noise.