I'll take another run at it...
Whether the rear sight of the HK is elevation adjustable or not, really isn't the issue as I see it.
Assuming the front sight only, post in a globe. If the post is centered in the globe, or whether the tip of the post is below or above the center of the circle circumscribed by the front globe sight, the post will subtend X minutes of angle from the theoretical center of the globe. This subtension, as measured in MOA on paper downrange will remain the same, no matter the distance, as it is a static relationship.
Thus if you zero your rifle at, say, 200 meters using the globe centered in the rear aperture, and set the target on the tip of the post, and you raise elevation your standard come-up for 300 meters, say 3.5 moa for a 300 meter zero, and maintain the same relationship between the front globe and rear aperture, and the target on the tip of the post, it will hit to zero.
The relation between the sights, as measured in moa of deflection, will remain the same, irrespective of distance.
Or did I miss something?
It's hard to explain, but If you center the circle within a circle without the post being in the center on both the x and the y axis it will be off. If you look at my drawing it will show why. Without the tip of the front sight post being centered in the rear aperture The line of aim runs ALMOST parallel to the bore. Most people using this type of sight will shoot low. at 50 yards when i aligned the circles i shot about 5" low, and no matter how much i adjusted elevation it still shot about that low. It's quite difficult to explain.
I made another diagram, this time in paint. The 2 circles are the front sight hood, and the rear aperture. The red line is the actual line of aim and the green line where the line of aim should be.
Im quite talented at making images in paint, can you tell