Depending on facial anatomy, the back of a lens can be anywhere from 5 mm to 17 mm away from the front of the eye. This value, known as the back vertex distance, has a drastic impact on how the visual field maps to a pair of glasses.
I find the easiest approximation uses an inverse tangent function to calculate the number of degrees.
When calculating how the visual field relates to glasses, I find it helpful to consider the visual field’s origination at the nodal point. A typical approximation of the nodal point is 7 mm behind the front of the cornea. Adding this value to the back vertex distance gives us the “adjacent” length. Our “opposite” is 1 to find the number of degrees for 1 millimeter. If you used 6, it would tell you the number of degrees for 6 millimeters.