90* is the reference point, however as you have noticed a 0 degree or a 90 degree stem(same thing) may not put you at a horizonal position because the steerer on the fork is at an angle as well.

Also, all stems(other than quill) can be flipped. So that 84* stem is also a 96* stem aka +/- 6 degrees. If you want another stem and want it to be oriented the same way as your current stem, you will need the same angle (80*).

So the question is...Why are you getting another stem if you like the one you have?