Originally Posted by

**Darryl Meister**
If you are referring to the tilt due to meniscus decentration, Stimson acknowledges the effect in *Ophthalmic Dispensing*. I derived my own formula, however, many years ago for my old Rx compensation software, using the geometry of my figure and operating with the assumption that a modern, guided mini-bevel is generally positioned equidistant from the front surface of the lens.

Also, you would use the arcsin function in this formula, not the arctan function, since you know the distance DEC and the length R_{1} to FP. It is difficult for me to explain without a drawing, but the arctan function would only apply if the distance DEC was measured in the plane of GC, with R_{1} extended past the surface, not in the plane of FP.

That said, for the small angles involved, the dfferences between the two are negligible and, in fact, you could drop the inverse function altogether and simply compute the angle in radians as the ratio, DEC/R_{1}.

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