The key difference is in your posture - with ErgoPrism Loupes, you're not bending your neck or back to see your working area. This means that your old working distance measurement will not work for ErgoPrism Loupes because your body will be in a different position than it was with traditional Loupes.
Because of the improved posture, working distance with ErgoPrism Loupes is typically 2-4" farther than with traditional Loupes. The exact distance varies from person to person depending on several factors, therefore it's not possible to just add a few inches to your traditional working distance to get your ErgoPrism working distance. You'll need to get a fresh measurement with your new and improved posture to make sure your ErgoPrism Loupes get made the way you need.