Grr, post eaten.
According to Therlun, past 170 AD crit is a more gold efficient buy than straight AD. Therlun may in fact be lying about this just to fuck with us. Because he is a fascist. I donāt know where the math comes from. Assuming this is correct for a moment, finishing PD from Zeal is roughly equivalent to buying a BF sword. So Cait should finish her PD. PD is a major DPS booster since it adds like 20 crit and 35 AS over Zeal (the move speed is just icing. Delicious, chocolate icing). At this point I am at 1.5+ attack speed, 55 crit chance, with 2.5 damage crits. Lovely.
What you do next, and sometimes what you do from Zeal, is a little more complicated. Yes, thereās times where BC is better than LW. LW scales much better, though itās possible the enemy will never stack armor until that point (if they stack health you may need to adjust accordingly). None the less, itās cheaper and I usually go with LW after PD (and Iāve done BC on her all of once), before BT.
Sometimes before finishing PD you need to get more Armor Pen. Itās a very situational thing. But maybe the real problem isnāt Vayne or Annie but Lee Sin. If itās Lee Sin,and he has Atmas and Wriggles, heās probably sitting on 120-130 armor. LW will probably make you kill him in 4 seconds. Iāve swung games in this fashion.
So I go IE, Zeal (both the exact order there is really situational, but I sometimes have zeal before IE is finished), finish PD (usually), LW (usually), BT. But I usually have defenses before BT is finished (or started) and itās rare a game goes that long. Iām totally ok with relying on LW for pen purposes even when enemy armor is in the 80-100 range. Which it frequently will be, even on ānakedā carries.
Lastly, I fooled around with a flat-pen build on MF for a little bit. Reds, Quints, masteries is like 31 pen or something. Brutalizer, BC, Starks and youāre at like 110+. But I felt like this build was especially dependent on getting out to a farm/game lead. Kind of hilarious though; people look at you and figure your damage as being too low, not considering that you are practically doing true damage or are doing true damage. No reason Cait canāt do this, but it never felt like a SRS build. Also, I have trouble imagining not using flat AD quints at this point.