>Have you considered doing a slightly different scale that accounts for the diffraction of the glass...

Yes, that is a possibility and I have done the mathematics to handle the refraction of the beam (somewhat complicated because of the angle if incidence changes with the change in elevation of the sun). There are two reasons the outside engraving is less desirable:
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1.) As the angle of incidence gets larger, the amount of reflected beam grows larger to the point where the sun spot image becomes lost in amient light.
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2.) Exterior engraving is subject to wear and tear in operating the instrument.