With regard to the percentage of 5 star ratings, I'm not sure that there should be a "quota" or "target", although I do agree that they should be the exception rather than the norm.
I think I have reviewed ten 2010 albums so far this year. One had a five star rating, David Minasian, and I do consider this album to be exceptional.
I have, from what I can make out, another five to review, a couple of which I have been holding back. However, one thing is for certain, and that is not one of them will be five stars. There is a two star, then a mix of three & four.
So, one out of fifteen, or just short of 7%. That's higher than some figures quoted as a "guide", but still only one album for the entire year's purchases.
I also consider a three star rating to constitute a good album, and definitely worthy of purchase. For example, i reviewed Blackmore's Night new album yesterday (I think) and gave it three stars. It is a good BN album, and definitely one that fans of the band and that type of music would enjoy, but is by no means essential.
Finally, I don't compare albums between artists when awarding ratings. They are awarded entirely on that album, and that album alone. It is also, of course, possible for artists/bands to have more than one masterpiece. Yes, Genesis, and Marillion (I know that's my personal bias
), for example, have certainly produced these.