The initial tension for guy wires for guyed towers is normally 10 percent of the breaking load of the tendon, with upper and lower limits of 15% and 8%. This is based on the TIA/EIA-222-F "Structural Standards for Steel Antenna Towers and Antenna Supporting Structures". Assuming that your 7/16 in. guy is EHS 7-wire strand (extra high strength), the breaking load would be 20,800 lbs, so the preload would be 2080 lbs.