http://www.gamesdiner.com/GULLIVER/B5combat.htm#DeclaringDefenses suggestions by Tbone about declaring defenses before or after rolls.
Non-preemptive defenses (as GURPS normally works, no defenses are used against misses) is described as "wait and see" and "panic defense" and "last-second defense"
He suggests -1 for melee, -2 for thrown, -4 for arrows, -8 for bullets, -16 for hypervelocity
He describes it as "represents actually dodging or blocking the bullet itself, not jumping out of the muzzle's path"
However the problem with that, is there should be a distinction between "dodging an aim I know is aimed directly at me" vs "an aim I know is aimed in my general direction" and there should be statistical differences between reacting to those different ideas