I, and others have suggested this before -- you should review those threads.
If you do this, 5-10% is too high. Also, there should be a minimum ... say, 5-10% OR over 5-10 feedback. Why? Because I have seen too many times where a newbie joins a game and either makes a mistake out of honest ignorance or has the bad luck to play WITH a real jerk.
The worst offenses are those who seem to think that they can just decide that certain games are just supposed to automatically be "real time". I mean, if you insist on real time then PAY the $25 and play speed games! The other group are those who decide they can decide how everyone else should play. Interestingly, these are almost never even lueitenants, never mind majors or above. Often they criticize the person who won (like gee -- maybe they had a REASON to ignore your idea of play???)
Some tolerance is definitely appropriate. Having a few games under your belt doesn't give you the right to blast every new person who either misreads or just doesn't understand the rules fully ... a nuetral or even just a politely worded pm goes a lot further. EVERYONE has a learning curve ...
I tend to read negatives. In some cases, it is clear that someone might have had a bad attitude or bad luck (server problems, so forth) initially, but then has LEARNED THEIR LESSON. A few even have "wonderful" friends who think it "fun" to give negatives. Usually, I am as likely to put those LEAVING the negatives on my ignore list as to put the receiver.
AS for the newbie exclusion filter ... this has been discussed ad infinitum. If you can't be bothered to read it, there is no sense even commenting, but I will say that EVERYONE starts out new, many go down to cook after their first few games because, well, we all have to learn. If you win, you gain a few points. If you lose -- well, you DESERVE to lose! Its called open competition.