A good metric for measuring in-game performance

I posted this on Reddit recently, and I am aware that many of you hate Reddit links, but I wrote this post already, and I don't think it's a good idea to write it again. So here's the quick summary. I came up with a metric of in-game performance (-100% to +100% each game) and a program to compute this for you based on your match history. Let me know if you want to know how you've been performing recently (give me the # of games in Ranked solo Q you want to base this on). Now you can see if you should be gaining ELO but are not, or if you got carried to where you are now (a bit sensationalized, I admit) but this metric has been pretty solid so far, especially on sets of 20+ games. The details are on the Reddit post, if you are interested in learning more about the background or about the calculation.

Now, the first thing you will think of is "WTF THAT IS WHAT MMR IS FOR!!11!!" and I agree that is very logical, but this is more effective in the short run, while MMR is better in the long run (read the post in the link). I do lightly touch on other issues with this, too, but if you see an egregious problem with the way I do this, please let me know (and a possible remedy to aforementioned egregious problem would be very helpful!).

tl;dr Let me know if you want an estimate of your performance and I can compute it for you.