Overall experience results in the rating. The calculation of epic heroes drop at exactly 5% nothing more nothing less. In a pull of 20 you are guaranteed 1 epic, this means in order to get epics higher you would have to perform hundreds of pulls. Not to mention the fact that the rewards given are not remotely enough to achieve this. Also the quest and handholding I'd also very misleading and it seems this is done on purpose. Bad karma noted.