It's NOT a correct calculation you are using:
50 feet height = 1000 feet or 300 meter
100 feet height = 2000 feet, 600 meter or 0.3 nm.
All in round numbers of course.
Height (feet) = tan(glide path angle) x distance (has to be same unit, feet too).
Unless you can provide a reference FROM JEPPESEN (don't have Jeppesen available to look it up), I'll maintain that "Beyond the glide slope", is just a measurement of the distance from the antenna to the end of the runway (landing roll available, so to speak).
"As discussed previously manufacturer data is normally based on a threshold crossing height of 50 ft and a touchdown point that may not be achievable in line operations."
Normally = unsloped (flat) runway. And 3 degs glide slope. Steeper glide and upslope you pass the threshold higher, but still touchdown 300 meter down the runway (assuming no flare). Touchdown point decides your landing distance, not threshold crossing height.