I've tried helping people out for the Gargoyles boss at levels 11, 12, 13 and all the way up to 20 and it seems like it's just SUPER rare to get someone who needs help (as opposed to invading…)
At level 11 or 20, the 10% level difference is only 1-2 levels which cuts you out from helping others of VERY similar level.
What's a good level for that boss and some other common bosses that people have trouble with, like 4 kings?
Best Answer
There's a really good image on the Dark Souls wiki for this exact thing.
http://darksoulswiki.wikispaces.com/file/view/1323119782398.jpg/288072208/873x671/1323119782398.jpg
This will let you know what levels you should be between for there to be a great sunbro. Using this, I was able to get around one summon every half hour from things like Smoug and Orienstein. The level bands are pretty easy to hit, until you get to the Lords or things that can be fought at a larger band of levels (like Sif or Pinwheel). Those areas are much more about when the player takes on the current boss. You will get very variable level bands here, so it's very hit and miss for those areas. But this should help you all the way up through Anor Londo.