Welcome to the world of "exterior ballistics", or simply put, what happens with regard to a bullet's flight once it leaves the barrel.
It's about bullet weight, bullet shape (form factor), and bullet diameter. In the case of a .45 ACP cartridge, velocity is a nominal factor, so I'd just forget about it in your case.
Those three measurements (shape, form factor, diameter) determine the bullet's "ballistic coefficient." The bullet's "ballistic coefficient," determines the bullet's trajectory -- in other words, it's arc in flight. If I could sum it up more, I'd say that "ballistic coefficient" is a bullet's ability to overcome the friction of air resistance in flight. The .45acp has what is often termed as a "rainbow trajectory." A good reloading manual will list some coefficients for a given manufacture's various bullet shapes and weights.
When target shooters shoot at long ranges, all of this becomes quite important to them in acquiring exact bullet placement on a target, but with .45 ACP, most of us ignore all of this hokey pokey and sight our pistols in for realistic self-defense ranges. Anything beyond 25 feet, and a smart district attorney, or prosecuter, is simply going to have a field day making a case that you could have avoided the shooting somehow anyway. It's more important for people like "us" to practice shooting tight groups in the center-of-mass of a silhouette target. *smiles*
So, try to think of your bullet making an "arc" in flight. When you started shooting at the longer distanced targets, you found the spot in your bullet's flight path (trajectory) where it's close, or at, the top of it's rainbow curve in flight. If you had continued to increase the distance, the bullet would eventually start hitting dramatically lower. On the other side of the coin, if you shoot at "point blank" range, then expect your bullet to hit 3/4" low because that is the distance from the top of the front sight to the centerline of your bore on your average .45 ACP semi-auto pistol.
Unless you're a target shooter, I'd sight in the pistol for about 25 feet since that is about the range, or even less, where most self-defense experts say you're gonna be firing at a perpetrator. I hope I haven't made all this even more confusing for you.
And if you're an algebra fan: Ballistic Coefficient: C = w/id2, where: c = ballistic coefficient, w = mass, in pounds, i = coefficient of form (form factor), d = bullet diameter, in inches.
