11-4.2 Print Reflectance Difference The Postal Service requires reply mail pieces to have a PRD of at least 30 percent. The equation for PRD is as follows: PRD = (RW – RP) x 100. Note: “RW” is the reflectance of the background (e.g., the envelope or card), and “RP” is the reflectance of the ink (e.g., character stroke).