That just gave me "tired head".
Since if came from you, I'd like to understand what the hell you're talking about because I know I could use it.
Sorry about that, JLP. I'll try to explain it a bit differently for ya.
Since the scope is mounted above the bore, the slug comes out below the line of sight (the line of the crosshairs of your scope projected forward). The slug comes out on a slightly upward trajectory and first crosses the line of sight at somewhere around 20-25 yards from the muzzle. Then, it will remain ABOVE the line of sight until it comes back down to the line of sight at say 125 yards (known as your "zero distance").
During the first 20-25 yards of flight, the slug will be below
the line of sight by about 2.5" or less. From 25 out to 125 yards, the slug will be above
the line of sight. At 125 yards (in our example) the slug will have dropped back down to the line of sight.
IF we zero the scope at such a distance (say about 125 yards) so that the MAXIMUM height of the slug above the line of sight is 3" at about the 2/3 point (roughly 75 yards), then the slug will be no greater than 3" above or below
the line of sight all the way from the muzzle to the zero distance (say 125 yards). For shots greater than 125 yards, the slug will be below the line of sight. Any shots taken at greater than 125 yards will require you to hold over the intended point of impact. The amount of holdover will depend on how much farther than 125 yards the shot is.
So, to use this method, set up a target at say 125 yards and adjust your scope to hit the bullseye at that distance. Then, move your target to about 70-75 yards from the muzzle, aim at the bullseye and see where you hit. If the slug hit higher than 3" above the bullseye, then you need to move your "zero distance" closer than 125 yards. If the slug hit less than 3" above the bullseye, then you could move your zero distance greater than 125 yards. The final zero distance you end up with will depend on the ballistic properties and initial muzzle velocity of the slug. A fast sabot slug will give you a longer "zero distance" than a slow full bore diameter slug.
I hope this explanation helps. If not, take 2 Budweisers and call me in the morning.