increase disk space slack recommendation

As discussed in #84 and #116.

guide/install.md

@@ -183,19 +183,21 @@ In the user interface,
       each camera.
 
 3.  Assign disk space to your cameras back in "Directories and retention".
-    Leave a little slack (at least 100 MB per camera) between the total limit
-    and the filesystem capacity, even if you store nothing else on the disk.
-    There are several reasons this is needed:
+    Leave a little slack between the total limit and the filesystem capacity,
+    even if you store nothing else on the disk. 1 GiB per camera should be
+    plenty. This is needed for a few reasons:
 
-    * The limit currently controls fully-written files only. There will be up
-      to two minutes of video per camera of additional video.
-    * The rotation happens after the limit is exceeded, not proactively.
-    * Moonfire NVR currently doesn't account for the unused space in the final
-      filesystem block at the end of each file.
-    * Moonfire NVR doesn't account for the space used for directory listings.
+    *   Up to `max(120, flush_if_sec)` seconds of video can be written before
+        being counted toward the usage because the recording doesn't count until
+        it's fully written, and old recordings can't be deleted until the
+        next database flush. So a 8 Mbps video stream with `flush_if_sec=300`
+        will take up to (8 Mbps * 300 sec / 8 bits/byte) = 300 MB ~= 286 MiB
+        of extra disk space.
     *   If a file is open when it is deleted (such as if a HTTP client is
         downloading it), it stays around until the file is closed. Moonfire NVR
         currently doesn't account for this.
+    *   Smaller factors: deletion isn't instantaneous, and directories
+        themselves take up some disk space.
 
 4.  Add a user for yourself (and optionally others) under "Users". You'll need
     this to access the web UI once you enable authentication.