Here are the 2 web sites I got the information from with quotes exactly from each web site. Yes, it has to be a major in the box and at landfall in S. FL. Only 2 majors ever to hit S.FL. missed the box, Andrew barely and the '35 storm. And if it is a major to miss the box it also almost always misses S. FL.

1-It's called the Hebert Square. Almost every Atlantic storm that has struck Southeast Florida as a major hurricane has gone through this small box in the ocean, and almost every major storm, of at least 111 mph, that has bypassed it has missed South Florida.
http://www.palmbeachpost.com/storm/content/weather/special/storm/reports/070603box.html

2-Nearly every major Hurricane that hit S Florida since 1900 passed through these boxes. When major Hurricanes miss these boxes,they virtually always miss South Florida. If a major Hurricane moves into these boxes South Florida really needs to watch out. These boxes approx 335 miles x 335 miles includes the Virgin Islands but not Puerto Rico. The pattern has proven accurate for 9 out of 10 storms storms that developed & hit Dade, Broward & Palm Bch Counties.
http://www.hurricanecity.com/hebertbox.htm