@OldVet , I'll take a stab at trying to explain this. Sounds like you have it backwards. In a scope with a multi-dot reticle, the center where the main crosshairs intersect should be the aim point for zero at your chosen distance. Aim points, such as dots or tics that are below the center crosshair intersection, will add elevation to your shot, causing your shots to impact higher. Aim points above the center intersection will add declination, causing your shots to impact lower. Different shooting conditions require different aim points.
Shots that require more elevation to be on target require use of aim points below your zero (holdover). Shots that require less elevation, most commonly when shooting at angles up or down, use of aim points above the zero (holdunder). If you've ever been frustrated by trying to hit a squirrel up in a tree because you miss every time, it's because gravity is having less of an effect on the projectile arc and it is impacting above your line of sight. That is one common situation where you need holdunder or negative elevation or, in other words, an aim point above your zero aim point. That's why there are mil dots above the center crossing in a scope reticle. If you are using iron sights or a single dot site, you have to do this manually. When shooting upwards for instance, you need to aim below your desired point of impact.
Choosing your zero distance is a highly individual selection. Depending on the height of your sight above your barrel, and velocity, setting a zero distance that's relatively short compared to the projectile arc, will result in two crossings: The point of aim (POA) and point of impact (POI) will be the same at your first zero distance. For some distance beyond that, the POI will be above your POA as the projectile peaks out in its arc, and then a little bit beyond that, they will coincide again, as the projectile curves back downward and crosses your line of sight, giving you effectively two zero crossing distances.
At distances between the 1st and 2nd crossing, the projectile will be above your line of sight and you may need some declination/holdunder, an aim point above the center crosshairs to compensate. At distances shorter than the first crossing and longer than the second crossing, you will need elevation/holdover, and an aim point below the center crosshairs to compensate.
I tend to prefer to choose a zero distance that coincides with the peak of the projectile path so that there is only one crossing. All distances shorter or longer than this zero distance require only elevation/holdover. The only exception would be when shooting upwards at steep angles.
Hope this helps. Remember that your line of sight is perfectly straight but the path of the projectile is an arc as gravity works on it during its flight. I'm sure there are some other threads with illustrations that may be helpful for you.
Regards,
Feinwerk
