The basic issue here is that of you don't define a proper system of coordinates, you are screwed.
As soon as you start sayings things like "its an increase in this case but a decrease in another case" you should be getting a sense of a poorly constructed coordinate system.
Unfortunately we have this in aviation.
e.g. write down the equations for a climb, as taught in your typical pilots ground school.
they are based on climb angle.
Repeat for the descent.
they are based on descent angle.
If you compare the equations you will note they are exactly the same except for the presence of a minus sign anytime the angle crops up.
In otherwords you couild just have one set of equations, say for the climb, with the proviso that if the answer comes out negative your climb is negative (i.e. a descent).
This thread is a good example of non standardisation and the need for professionals in any technical field to have a decent scientific education.
Unfortunately we don't have such a thing any more.
CPB