So I still seem to need a DSM file on my SD card or on internal memory for Terrain Follow to work.
To get that
- I went here: https://apps.nationalmap.gov/downloader/#/
- select the 1/3 arc sec DEM (just right density for M300), click search products and download it (geotiff).
- use your preferred GIS to clip it down to ~4sq miles (to fit within the M300’s max dsm file size).
- upload the geotiff to your controller and select that file.
it will calc it out and give you a color spectrum on each flight line.
NOTE: I have only planned with this so far… first Terrain Following flight is early next week… 
That said this is the DSM I have been using on our phantom drones (via Map Pilot).
EDIT: make sure you get an area a good bit larger than your flight area… I’d hate to think of what would happen should your drone ‘fall off’ the edge of the DSM surface… 