lol ur formula is so fail so if they have 20 amor is 20/120 reducing 84% , and if they have 10000 armor will be 10000/10100 = around 1 so no reduction...go back to school learn some math.
formula is Physical damage = Damage * [100/(100+armor - armor pen)] .
anyway math speaking quintessence flat damage is superior from armor pen items cuz they scale way superior mark/quintessence wise. overall flat damage is superior to armor pen. on lvs 1-5 and give u HUGE HELP on last hit cuz minions have 1 armor. from lv 7 is better armor pen.