Sorry, but a 3/7/8 adc IS feeding if the games 20 minutes long. They just got some revenge. And if they gave a Jax 7 kills, the games over. End of story, they fed. And I'd also argue there are times an adc ends up like this because the teams lacking peels and is chasing while an assasin comes in and picks them off. That's not the adc's fault and it's not a mistake they made, but one their team made.
Everything's situational. 0/16 should be banned from rank for 5 games. 3/14 (as I see consistently) is something unforgivable. I'm as patient, helpful and good as the next guy. But I can't carry a team that gives up a death a minute no matter how often I gank, how much pressure I put on the map or how many objectives I grab. I tend to pack on 2-3 deaths in every game at the very end trying desperately to hold onto a losing game. Typically it's 1v5 under tower. So my 21/10/18 Trundle was really a 21/4/18 until the very end where my team ran up middle without me into 1v5's until I couldn't kill anyone and got killed under tower. And the 9/4/9 Nasus game with 600 stacks where I could 2 shot their adc was really a 9/1/9 until I was trying to 1v5 under nexus towers. Those aren't my fault. There isn't a mistake -I- made. There isn't more I could have done. It's my team mates that threw and threw hard.
The truth about League is that guide is BASIC shit everyone should know. I tell people to cs, to help me gank, to grab objectives, but there's a point at which I don't believe (and I think it's absurdly true in this pre-season) that you can't recover from the math. The deck is stacked too far to recover from. And I tell everyone I can that over 5 deaths is feeding. And it is, that's roughly 10k gold to just hand to the other team that really didn't need to happen. That's a 3 item lead and probably 2-3 levels and 20-50 cs. I try to explain, calmly most of the time, why things are bad and what they can do to change. I tell the 0/12 Blitz to sit on my ass or tower and do nothing but q and run. But instead he runs straight up mid into 1v5 to die again. I can't help that kind of special person... I'm not saying this guide and what you said isn't a good rule of thumb, I just think there's certain truths that bronze and silver players refuse to admit. They are missing absurd amounts of cs, dying too much, out of position, or pushing when they should clear and defend. It's simple things and they're quickly abandoned the second they see that they can go superman into near impossible odds because they might come up with one kill. Like that Yi that was 12/14 by running into their team 1v5 with everything popped and alpha strike out their adc or mage and die instantly... and mostly he got the kills at the end... These plays are the norm from what I see. And it's like no one cares.
