Horizontal eccentricity

Horizontal Eccentricity refers to the horizontal axis, measured in degrees, along the visual field. The blind spot is defined along this horizontal axis.

The blind spot starts at a certain eccentric angle (temporal), and finishes at a certain other angle further out from the centre of fixation (fovea). These are the two angles you are calculating. (For the size of the blind spot you have to take the difference between ArcTan (s/d1) - ArcTan (s/d2): d1 being the distance from the observed surface where your scotoma starts; d2 where it ends; and s being the difference betwixt the two.)