I'm sorry but where are you getting these values from, and how are you even able to state a static % increase in damage for a piece of gear with attack+? The % increase from justiciar's will be entirely based on how high the defense is of your opponent. If you've already capped the differentiation then +attack will do nothing for you at all, whereas +agi will always give more damage.
I just simply plug the numbers of my current build vs a mob 6 lvls higher that has 450 def (generic mob slightly higher lvl). You'll not cap ratio without some serious buffs going on or the mob being ep.