Defensive Stance is calculated after the mitigation from armor. If an unmodified attack hits for 100, and you have 60% damage reduction from armor, you take 40 damage prior to Defensive Stance. Defensive Stance removes 10% damage, thus you take 36 damage total.
Basically, in order to figure out what damage you take, calculate (1-DR% from armor) * (1-DR% from Defensive Stance)
In the above case, that'd be 0.4 * 0.9 = 0.36. The actual damage reduction after armor and after Defensive Stance is thus 64%.
ಠ ,ಠ Dovie'andi se tovya sagain - it's time to roll the dice