If I understood what he was saying correctly then I'll use an example that will hopefully make it clear. Picture a blank wall, on the wall mark a point with a pencil at roughly eye level. Get a lazer pointer, hold it at waist height and stand back 10 paces and make a note of how much an angle you have to hold the lazer pointer to put a dot on the sight. Now step forward so that you're only a couple feet from the wall and point the lazer pointer at the dot. In order to hit the convergence spot (the wall) you have to hold the pointer a higher elevation/angle the closer you are.
Dang almost sucked me in with that one. Had to think about it and do some research.
In the image your laser line is the 'Line of Departure'. Notice the 'sight height' this is the same as the difference between your eye level and waist height your holding the level at; it is also equivalent to the height difference between the cockpit sight and gun mount location. The convergence setting is 'Zero' on the illustration. Assume a 6' sight height and 300yd range and the sight depression angle is a miniscule 0.4deg. Most importantly notice that the bullet trajectory is downward shortly after 'Mid-Range Height'.
Think about setting the Zero point out to 400 yds. The line of departure would have to be raised to compensate for additional bullet drop in order to intercept the level line at 400 yds. The mid-range height would now be at 200yds. Meaning that you are lofting your shells higher with a longer convergance setting.
Back to way your example is so seductive. Move the Zero point in along the
sight line. The line of departure would have to be lowered the closer you got, up to the point that the trajectory is tangent to the sight line. After that (like in your example) the line of departure would have to be raised as Zero got closer because the bullet is still on a rising trajectory. The range to that tangent point depends on Sight height, depression angle, and bullet trajectory all of which vary with the various airframes, but I'm willing to bet is closer then we usually engage at.
I'd be intersted to find out about the actual mechanics of setting convergence and sight depression on our a/c. I'm think'n its a lot easier to level the plane (usually a somewhat abriturary but consistant reference like seat/canopy rails, engine rails, marked rivet heads on fuselauge...) set the target out at the convergence distance then adjust the guns to hit it and adjust sight depression to match. Notice in this method the zero point (target) moves along the
level line. And the bullets would never hit on a upward trajectory.