I have heard the goal of 85% before ... not sure the original source, and maybe it's just FT hearsay, but Googling "TSA Precheck Goal Participation" I did find
this story at the top of the list which discusses some more short-term goals:
TSA Administrator John Pistole set an end-of-2013 goal to funnel 25 percent of all travelers through speedier security arrangements, which includes PreCheck and gentler processing for those 75 and older and children 12 and younger (both populations can leave their shoes and light coats on). He hopes to goose the number to 50 percent by the close of 2014.