What does H/6400 mean?
This is a designation of the underlying math of how the rangefinder works. If you extrapolate the points in the rangefinder all around you, they will form a complete circle around the viewer. For this circle a radius of 1km was assumed, making the circumference 6283m which rounded up makes 6400, a number easily dividable by 4. This rounding up was found to barely change the distance outcome but makes it drastically easier to calculate distance. After the war this method of rangefinding was adopted by NATO.
How to calculate the distance to a target?
To find the distance to a target first we need pick an object that we know the size of. This will be our reference. As you can see each line represents 5 points.
If we pick this fence pole as our reference we can see that 1,5 lines fit in the height of the pole. This is equal to 7,5 points. A fence pole has a standard height of 1,2 meters.
Now we multiply the known height of 1,2 meters by a thousand and then divide by the number of points we found. This gives us a distance of 160 meters.
This method works for all optics that have the H/6400 rangefinding capability.