(Disclaimer: I was studying for finals like a madwoman when I made the above post.)
Okay, so in optometry there are a lot of things that are either upside-down, mirror-imaged, opposite or 90-degrees off because of optics.
After awhile you find yourself saying things like, "well if the vertical meridian is at 120, that's 30, so that's the horizontal focal line," and it sounds normal to you.
Also:
-- It's on the nasal retina (right side)? Well then it's in temporal space (left side).
-- It's the patient's 10-degree meridian? That's the observer's 170-degree meridian... which makes it the 80-degree focal-line.
-- He's a myope? The far point is in front of the retina so you subtract the BVD. Hyperope? The far point is *behind* the retina so you *add* the BVD.