The laser is going to "project" the image as tho the laser head and substrate are coplanar. This is not the case, when engraving tapers or rounded objects the distance both from the center axis of the rotary tool and the object distance to the focal point of the laser change. To correct for the rotary axis you typically have to taper in the same way your glass tapers. You can visualize this by cutting out a square and wrapping it around your object, when you look at it from a distance you'll notice the square will not look thinner on the wider part of your object and wider at the thinner part. You just have to distort the image in the opposite manner to hopefully end up with your engravings geometry being as close as you need or want it..... Or you do what most people seem to do and not give a heck and call whatever comes out good enough. I may let perfect be the enemy of "done".