http://www.gamesdiner.com/decide-drop-excess-combat-info-from-defense-evaluation-gurps-4e3e
House rules regarding issue of choosing whether or not to defend based on attack roll. Ie you coukld opt not to dodge a burst with just 1 bullet hitting.
too all or nothing though. Cole's Dodge This! has a similar problem.
One solution could be to use Scatter rules with perception rolls. Even if you don't correctly perceive the target of an attack, the margin of failure should determine where you think it should hit.
However, since mistakes can result in accidental success, the 1d6 for direction of deviation should be rolled separately for each yard.
There should possibly be 2 rolls. First to know that an attack was launched at all "he's shooting! everyone in the room duck!" and a second to know the target of the launch.
This is especially important with things like sacrificial parries or dodges. You need to know whether ally A or ally B is the one being shot at. Cole and Tbone's approaches mostly focus on the central idea of only reacting to attacks against yourself.