The NEI-sponsored Ocular Hypertensive Treatment Study (OHTS) found that lowering intraocular pressure (IOP) by at least 20% produced a 50% protective benefit for individuals who had an elevated IOP but no optic, disc or visual field deterioration.
Kass, Michael A., Dale K. Heuer, Eve J. Higginbotham, Chris A. Johnson, John L. Keltner, J. Philip Miller, Richard K. Parrish, M. Roy Wilson, and Mae O. Gordon. The Ocular Hypertension Treatment Study: A randomized trial determines that topical ocular hypotensive medication delays or prevents the onset of primary open-angle glaucoma. Archives of Ophthalmology. 2002; 120(6): 701-13. http://archopht.ama-assn.org/cgi/content/abstract/120/6/701