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FY 2012 Superintendents Book (page 61) refers to "Students identified for gifted services receive high quality differentiated instruction through the FUTURA Program in grades 4 and 5." and continues to state "The FUTURA program currently provides for 1,474 students in centers..." and then adds "In middle school, 2,207 gifted students participate in the SPECTRUM program offered during the resource block...".
So if you know the total number of students in grades 4 and 5, and then the total number in middle school, I am guessing you can calculate a rough percentage.