Once you open a raid night, like last nights full heroic clear you can select individual fights in the top. To view these percentages you would click on the Rankings button in the menu bar, same place you select the fight.
This takes you to a chart which analyzes the DPS/Damage done/healing/ anything you want to see, It defaults to DPS. On the right hand column labeled "Rank%" you will find your percentage.
This is the number we look at to determine where you fall, this number is found by looking at all other players of your class and spec and ranking them, then giving you the percentile you fall into.
Hopefully that helps.