Hi, the heightmap is just a 16 bit greyscale texture and sculpting means painting on that heightmap. But you can’t go below complete black 0 and you can’t get above complete white (somewhat around 65000). At the default z-scale complete black would be -256m and complete white would be +256m.
So you could change the contrast of your heightmap before importing it into UE (so that you stay somewhat above complete black and somewhat below complete white), and then adjust the z-scaling in UE.