I'd guess about 30-45%.

Although it's perhaps because I never get there really-really early and am left to wander around the crap bits but I've just got home from Grizzly Bear and there are quite a few grumbles about it. Be curious if anyone recommends any sweet spots to watch from. I found one near the end by the downstairs bar, looking across the steps downwards steps to get a clear view of the stage, head has to be tilted slightly to stop the soundproof glass sucking up all the sound tho.