20% protein is perfectly fine

I feed mine a feed that has 22% protein and have never had trouble with it, and never had a single case of angel wing.
What seems to cause angel wing is access to very high protein, high energy feed, such as turkey starter feed with protein exceeding 25%, that then causes unnaturally rapid growth in ducklings. Also, overeating plays a role, when the ducklings are eating more feed than they really need. So it's more a combined cause of both feed that's very high in protein, and overeating, which when combined cause the ducklings to grow so fast the weight of their wings outpaces the strength of the wings and the wing can't properly support itself.
So all that said, the feed you have should be fine. You don't want anything that's 16% or less protein, because that's not enough. They do need protein to fuel their growth. You just don't want them to be getting way too much.
What's also interesting is that males are more likely to get it, and it's more likely to be in the left wing. Some studies seem to show it's genetic, and then others show it's not. So what causes it is a little up in the air, and there are probably a lot of factors at play.
Here's a small article on angel wing and its causes:
http://www.metzerfarms.com/AngelWing.cfm