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The idea is to place the Zenni lens so there is good coverage over the fresnel lens, and then use the IPD in the prescription to place the optical sweet spot of the Zenni lens over the sweet spot of the fresnel lens. The calculation of 69 mm isn't really IPD, that's just the center to center of the lenses. IPD (interpupillary distance) is used in the grinding process of the prescription to establish that optical sweet spot in line with where your actual eyeballs are.