The problem with Average kills, deaths, and assists is that as you get better, so will your opponents, so the numbers won't change as easily unless you play other champions to balance it out (basically, lose with other champions then win with Xin because you're better with him).
You might want to try keeping track of something like CS (especially if you're laning him), since that's a metric that's much less dependent on your opponent's skill than average kills/deaths/assists.
Even that has problems though. End game CS isn't really useful. Rather, CS at specific time intervals would be better.
Anyway, just some ideas, because that's kind of what I'm doing right now.