The American Heritage® Book of English Usage.
A Practical and Authoritative Guide to Contemporary English. 1996.
3. Word Choice: New Uses, Common Confusion, and Constraints
§ 227. percent
Statistically speaking, you can increase a quantity by any percentage, but you cannot decrease a quantity by more than 100 percent. For example, once pollution has been reduced by 100 percent, it ceases to exist, and no further reduction is possible. In defiance of this logic, however, advertisers sometimes refer to a 150 percent decrease in lost luggage or a new dental rinse that reduces plaque on teeth by over 300 percent. You can usually figure out the general idea of these examples, but this doesn’t absolve them of being illogical. 1
Percent can take a singular or a plural verb, depending on how you view the quantity being described. Very often what determines the form of the verb is the noun nearest to it. Thus you might say Eighty percent of the legislators are going to vote against the bill or Eighty percent of the legislature is set to vote the bill down. In the second sentence the group of legislators is considered as a body, not as individuals. When you use percent without a following prepositional phrase, either a singular or plural verb is acceptable. For more on this, see subject and verb agreement under Grammar. 2