Barrier gains no benefit from 'any' DR. And best I can tell, contrary to what I would initially assume % stuff is counted before Armor. Armor is direct damage reduction, if you have 200 armor you are removing literally 200 from physical damage. With all the SnB passives and a Shield, from the front Cassandra can easily avoid vast majority of dmg. Coming from an angle though and she'll start taking actual damage.
In my current lvl 15 game she has less Armor then my 2H warrior (my char) but will only take 1 dmg often from the front while I'm taking 20-40. This is on NM, mind you. You got the defense which acts like %DR, she has 50% reduction from ranged (not ranged defense so its multiplicative... maybe), 20% general DR from the front. Has 176-ish armor from the Front.
If a ranged attack comes in at 400 dmg, lets say, that's cut in half to 200, then 20% is cut off bringing its base to 160, her armor is higher then that and she only takes 1 dmg. This is rather consistent. Even say 500 dmg, cut to 250, 20% from the front cuts off another 50 bringing it to 200. Shes only taking 25 dmg at that point which is a far cry from the initial 500 you'd see to Barrier (haven't seen 500, see 350-450 usually when hitting barrier).
That's gonna be the easiest way to tell how the DR and DR% are taking effect - see what the dmg is vs Barrier, then see what Cassandra is taking with out barrier on. That's how I've come to the basic conclusion that the %DR (defenses/passives) are going into effect 'before' Armor is. Which is far, far more powerful. As an example of that 400 dmg mark in reverse with ranged? 400 - 175 gets you 225, cut that in half 112.5, 20% off that and your taking 80 dmg. 80 dmg is a far cry from the 1 she actually takes in game.
What this ultimately means is all those %DR stuff, as low as they might seem make a massive difference to physical dmg when combined with armor. It ultimately makes your armor super effective. One thing I've noticed, and haven't had a chance to test it in full yet but +Armor% bonus seem to only effect Armor, not Front Armor. Which you might think supersedes Front Armor if its higher but... I dunno. I've seen me take less dmg from the back then the front after a War Cry, not sure if thats broken or im just delusional. Hopefully its fixed if its broken.