We need to know the distance around the bend. This is called the Girth or Arc. As all measurements, this can be taken to either the convex or concave face of the glass, but always specifies which. This measurement MUST be taken with a flexible tape. We need to know the radius of the curve you need. The radius is one half of the diameter, if this were to be pictured as a complete circle. If you are unable to obtain an exact measurement of the radius, then give us the chord and the depth measurements.
The Chord is the distance straight across the curve, measuring straight across from point to point. This measurement should be made with a flat ruler. To measure the depth of the curve, take one half of the chord measurement and at that point measure to the top of the curve. With the chord and the depth, we can mathematically determine the radius.