Tech has a page open.
Well my standard build is (besides being definitely different, but it suiting my style)
then I pick last one. So for my build, I need to break the 105-185 damage barrier to make it effective. And cost effect out lifesteal (I keep pd for kiting). Lifesteal aside, the damage boost is 56.7% (I tried to guess it, pulling calculator out now). 28.4% effective ad from Last whisper. That just leaves 28.3% to break armor wise to make it cost effective (percentage). Against someone who built no armor items and has no armor runes, they probably have around 50 armor (averaged since this is bot lane oriented). IIRC, 50% damage reduction at 100 armor, so 50 will be about 25%- taking 35% outta 50, 17.5, makes it 32.5 armor which is about 21 or 22%. I'd go into 90+ armor for breaking even mathematically with my build. I'd still probably end up with bt, so I might swap pd which brings a whole other setup of math. But to break damage, assuming no thornmail/gromp/etc, 90+ which ranges from cloth armor (high armor base people) to something like warden's mail at lvl 18. Meaning anywhere above bronze/silver where people don't go full glass mid/adc/other-carry its effective. But in bronze where you see builds like 6 ludens echo or as/ad marksman, I'ma stick with saying bt is effective. So yes, I'm wrong higher elo. =) I get that. But I built lw and realized "hey, my damage is crazy low" and decided to make this since people do cookie cutter builds a lot. =)
Edit: Tech is nacho. The other poster.