diff --git a/docker/pgosm_flex.py b/docker/pgosm_flex.py
index f02d884..803031f 100644
--- a/docker/pgosm_flex.py
+++ b/docker/pgosm_flex.py
@@ -51,7 +51,6 @@ def get_today():
               help='Amount of RAM in GB available on the server running this process. Used to determine appropriate osm2pgsql command via osm2pgsql-tuner.com API.')
 @click.option('--region', required=False,
               show_default="north-america/us",
-              default="north-america/us",
               help='Region name matching the filename for data sourced from Geofabrik. e.g. north-america/us. Optional when --input-file is specified, otherwise required.')
 @click.option('--subregion', required=False,
               show_default="district-of-columbia",
@@ -96,9 +95,9 @@ def run_pgosm_flex(layerset, layerset_path, ram, region, subregion, srid,
                     skip_dump, debug, basepath, input_file):
     """Logic to run PgOSM Flex within Docker.
     """
-    if region is None and input_file is None:
-        raise ValueError("either region or input_file must be provided")
+    validate_region_inputs(region, subregion, input_file)
 
+    # Ensure always a region name
     if region is None and input_file:
         region = input_file
 
@@ -161,6 +160,27 @@ def run_pgosm_flex(layerset, layerset_path, ram, region, subregion, srid,
     logger.info('PgOSM Flex complete!')
 
 
+def validate_region_inputs(region, subregion, input_file):
+    """Ensures the combination of region, subregion and input_file is valid.
+
+    No return, raises error when invalid.
+
+    Parameters
+    -----------------------
+    region : str
+    subregion : str
+    input_file : str
+    """
+    if region is None and input_file is None:
+        raise ValueError('Either --region or --input-file must be provided')
+
+    if region is None and subregion is not None:
+        raise ValueError('Cannot use --subregion without --region')
+
+    if region is not None:
+        if '/' in region and subregion is None:
+            raise ValueError('Region provided appears to include subregion. The portion after the final "/" in the Geofabrik URL should be the --subregion.')
+
 
 def set_env_vars(region, subregion, srid, language, pgosm_date, layerset,
                  layerset_path):
diff --git a/docs/DOCKER-RUN.md b/docs/DOCKER-RUN.md
index 6c3274f..b4918de 100644
--- a/docs/DOCKER-RUN.md
+++ b/docs/DOCKER-RUN.md
@@ -48,22 +48,54 @@ docker ps -a | grep pgosm
 ## Run PgOSM-Flex
 
 The following `docker exec` command runs PgOSM Flex to load the District of Columbia
-region
-
+region.
 The command  `python3 docker/pgosm_flex.py` runs the full process. The
-script uses a region (`north-america/us`) and sub-region (`district-of-columbia`)
-that must match values in URLs from the Geofabrik download server.
-The 3rd parameter tells the script the server has 8 GB RAM available for osm2pgsql, Postgres, and the OS.  The PgOSM-Flex layer set is defined (`default`).
+script uses a region (`--region=north-america/us`) and
+sub-region (`--subregion=district-of-columbia`).
+The region/subregion values must the URL pattern used by the Geofabrik download server,
+see the [Regions and Subregions](#regions-and-subregions) section.
+
+The `--ram=8` parameter defines the total system RAM available and is used by
+internal logic to determine the best osm2pgsql options to use.
+When running on hardware dedicated to this process it is safe to define the total
+system RAM.  If the process is on a computer with other responsibilities, such
+as your laptop, feel free to lower this value.
 
 
 ```bash
 docker exec -it \
     pgosm python3 docker/pgosm_flex.py \
-    --layerset=default \
     --ram=8 \
     --region=north-america/us \
-    --subregion=district-of-columbia \
-    &> ~/pgosm-data/pgosm-flex.log
+    --subregion=district-of-columbia
+```
+
+For the best in-Docker performance you will need to
+[tune the internal Postgres config](#configure-postgres-in-docker) appropriately
+for your hardware.
+See the [osm2pgsql documentation](https://osm2pgsql.org/doc/manual.html#tuning-the-postgresql-server) for more on tuning Postgres for this
+process.
+
+
+## Regions and Subregions
+
+The `--region` and `--subregion` definitions must match
+the Geofabrik URL scheme.  This can be a bit confusing
+as larger subregions can contain smaller subregions.
+
+The example above to process the `district-of-columbia` subregion defines
+`--region=north-america/us`.  You cannot, unfortunately, drop off
+the `--subregion` to load the U.S. subregion. Attempting this results
+in a `ValueError`.
+
+To load the U.S. subregion, the `us` portion drops out of `--region`
+and moves to `--subregion`.
+
+```bash
+docker exec -it pgosm python3 docker/pgosm_flex.py \
+    --ram=8 \
+    --region=north-america \
+    --subregion=us
 ```