UE4 is a bit of an oddball, in terms of importing scales.

512 is the height reference, if your world machine extent has a maximum height of 2048 (always pick multiples of 512!!), then your Z scale should be 400.

For orizontal scales it's the opposite: 512 (or 505?) as the reference, if you import a 2048x2048px heightmap, your X/Y scale should be 25.

And that's it, it more or less works in multiples of 512. It works like a charm.

I dont know anything about World Composition, but I assume previous answers are correct.

Also, remember that UE4 calculates the zero height halfway throught the range, as it goes from 512 to -512.

Assuming your maximum (possible) height in WM is 2048 meters, if you want your terrain to be automatically imported at UE's zero, you should set your terrain zero at 1024 meters in WM (bring your water level at 1024 meters by adding a constant generator to the whole terrain so that they add up to 1024).