"Fair approximation" means getting close to the correct or accurate answer, idea, or representation, even though it may not be completely precise or exact. Full definition
Think of those root - beer - flavored barrel - shaped candies you had as a kid, and you'll have a pretty fair approximation of the tested vehicle's color. (articles.orlandosentinel.com)
It is too bad that the record of Reagan's trial - and - error on that job is lost, but his effort is a fair approximation of the present conservative challenge. (firstthings.com)