Importing Heightmaps - Heightmap stuck in far corner

In photoshop I made a 300x300 pixel heightmap. Save as 16-bit with a windows byte order.

Then in Unity, I create terrain and set the heightmap resolution:

Width: 300 Height: 100 Length: 300 Heightmap Resolution: 513 Detail Resolution: 512 Control Texture Resolution: 512 Base Texture Resolution: 512

But then when I import the heightmap, it places it in the far corner and seems to only take up 1/4 of the terrain.

Any help would be greatly appreciated, ive tried altering the values with no success and I dont really understand why it is not working when I have set 300x300 for the terrain and the heightmap is 300x300 (if anyone could provide a solution and a good explanation, that would be even better!).

Thank you!

300 is the size of the terrain (in meters). The expected resolution of the texture is 513 (in the example you've provided).

If you reimport it as a 513x513 texture, it should work fine