nonagesimal in Kepler's parallax computation
It looks like the assumption regarding the 90° point is that the amount of the ecliptic visible is 180°. So the 90° direction is the halfway point.

The nonagesimal is on the ecliptic, so the altitude approximates the arc. The wording in the initial part has that correct, but then it goes on to say "the same as the angle" - this seems to be a mistake. The arc is directly related to the angle, yes, but it is not the angle.