I can never remember the bloody math behind defense.
I do remember these guidelines:
If you have 50% as much defense as the opponent have attack score, you will take 25% less damage.
100 attack against 50 defense, the attack will deal a 75 max damage (and half that as minimum damage)
If you have 100% (as much defense as) defense as the opponent have attack score, the attack will deal 50% less damage.
100 attack against 100 defense, the attack will deal 50 max damage (and half that as minimum damage)
if you have 200% defense as the opponent have attack score, the attack will deal 75% less damage.
100 attack against 200 defense will deal a maximum of 25 damage
Ignore the bad spelling, I am tired and can't be bothered 
Sincerely
~ Kongdej