OK, here's my latest thinking on my house rule:
- Normally, Shield DB is only applied to one attack per round, and that attack cannot be from the rear.
- If the character is performing a full parry, then the shield DB might apply to more than one attack, depending on the shield size:
--- Large shields (e.g., wall shields, full shields) can protect against up to three melee attacks per round, and against all standard missile attacks in addition (e.g., arrows, sling bullets, crossbow bolts), but all the attacks must come from generally the same direction.
--- Medium shields (e.g., normal shields) can protect against up to two attacks per round (melee or missile), but all the attacks must come from generally the same direction.
--- Small shields (e.g., target shields, bucklers, shielding weapons) still protect against only one attack per round (melee or missile).