The documentation links have already been posted but instead of just increasing the threshold I would rather advice you to check where this is comming from. The threshold is already pretty high, therefore you could also just turn off SC instead of increase the levels..
I use a 10% storm-control level and even 5% on access switches but using the no-unknown-unicast and no-unknown-multicast options. This could do the trick depending on your environment.
Imho SC is primarily to lower loop impact or to do a hard cut in the event of a loop if you configure the shutdown action. So when there is a loop you can expect broadcasts to be at a very high level which should be enough for this decision unless you have a broadcastfree zone 
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As said, this depends on your environment so you could check if someone is doing excessive unknown multi- /unicast.