terms
Doyle's got it, but While we're at it, let's use some terms.
Line of Sight is pretty self explanatory. Dead flat, like a laser, from your sight system to the target.
There will be two points where your bullet crosses Line of Sight; one where the bullet rises from the muzzle to the "initial intersect" with Line of Sight, and the second where the slug drops back down to the second intersect, which is typically called "the zero".
Middle ordinate,, max ordinate, or Mid Range, is the point where the bullet is at it's highest point on the arc between the two intersects, (II and zero). Mid Range is not halfway, but typically about 2/3 the way to zero.
After the slug passes thru "Zero" it will continue to drop, and when the drop is equal to the "Mid Range" that is typically called the "working range" of the cartridge. I have also heard that distance called "point blank" range, contrary to the common interpretation.
Some comments: Big scope bells, high rings, see thru rings, any high sight system, tends to work against a flat trajectory, though not radically, but enough to bug me, so I avoid such rigs, which have other disadvantages as well.
The old, sage advice for a 100 yd zero on "typical" deer cartridges was +3" at 100. None of my rigs are set up that way these days. I now have access to a 200 yd range, and shoot to "zero" at that distance. When I come back to 100, I find that most of my rifles will be 2-2.5" high, and that suits me fine. That limits my "working range" a bit, but keeps the bullet closer to line of sight at mid range, and at the distances I shoot, that's more important than 50 yds or so of more "point blank" . Before I had 200 yd benches, I zeroed 2" high at 100, and that worked pretty good too, but I was probably coming in a little low at 200 with the slower cartridges. Didn't really matter, as I rarely shoot at a deer that far anyhow.
I do zero modest cartridges "on" at 100. My .44 carbines, the 7.62x39 bolt rifle (and AK) are all on at that distance.