More Horizontal Progression please

[Ibi]

Vertical progression means you replace every piece of gear periodically (every other major patch in FFXIV). Horizontal means old gear remains viable in new content, and can be replaced by new gear.

I'm trying to get Nektulos-Tuor to explain his definition of it, since he's only providing very nebulous, sometimes contradictory statements, like:

If the item improves your character by 10%, and one improves it by 11-13% in certain situations its still a 11-12% update to that encounter. The procs in EQ for example were very powerful but there was enough choice that one wasn't "too" powerful over another in the same situation, but they all gave your character a great deal of power.

versus

Vertical means there is one best choice.

According to the first quote, one best choice was horizontal, as long as it "wasn't too powerful over another in the same situation". According to the second quote, from the same post, one best choice was vertical.

I was hoping he could quantify where the line for "too powerful" is.

Nektulos-Tuor:

If, on fight 1, item A is +1%, item B is +3%, and item C is +2%, but on fight 2, item A is +7%, item B is +3%, and item C is +4%, I assume you'd say that's horizontal.

If everything else was kept the same, but on fight 1, item A was +40%, and on fight 2, item B was +50%, I assume you'd say that's vertical? (Even though everyone else seems to agree that's still horizontal.)

How about if on fight 1, item A was +10%, and on fight 2, item B was +12%? Or 18% and 14%? At what point does the switch flip that makes it go from horizontal to vertical, in your definitions?