Secondary Defenses 4 (Source)

The general theory is that when you are attacked, the attacker's accuracy skill (plus modifiers for range, posture, etc) is compared to your defense skill (ranged or melee, depending on what you're getting attacked with) to see if the attacks hits or misses. There are so many hidden variables that it's probably not possible to figure out an exact calculation without looking at the code. If an attack hits, then your dodge/block/counterattack "evasion" skill has a chance to trigger (which one depends on what weapon you have equipped). A comment posted on swg.allakhazam.com regarding the dodge skill indicated that you get a 1% chance to dodge an sucessful attack for every +10 you have in the skill. So at +125 Dodge (the max you can have with bonuses from boxes and tapes), you dodge 12.5% of attacks that would have otherwise been a hit. It's probable that block/counterattack use the same calculation (although I don't know if anyone's actually done the testing)