90 pinion you must be measuring vertically, that would 0 or 180
87 "down" would be 3 degrees from horizontal, or to put it "negative 3". That means the angle BETWEEN THE TWO is 3 and that is the end of the story........
So either you mistated the measurements or ???
Where are you getting 12 degrees? In reference to what?
Are you measuring the 90 and 87 in relation to "plumb" level?