diff --git a/pi-gen-sources/00-teslausb-tweaks/files/teslausb_setup_variables.conf.sample b/pi-gen-sources/00-teslausb-tweaks/files/teslausb_setup_variables.conf.sample
index 768a2d96..a9c584ec 100644
--- a/pi-gen-sources/00-teslausb-tweaks/files/teslausb_setup_variables.conf.sample
+++ b/pi-gen-sources/00-teslausb-tweaks/files/teslausb_setup_variables.conf.sample
@@ -73,21 +73,12 @@ export SHARE_PASSWORD='password'
 # Notes on sd card and image sizes:
 #   * A 128 GB or larger sd card (or USB drive, when using Pi4) is recommended.
 #     The minimum supported size is 64 GB.
-#   * When specifying sizes for the recordings and music drives, you can use
-#     specific sizes (e.g. 24G) or use sizes relative to the available space
-#     (e.g. 10%). It is recommended to use specific sizes.
-#   * The setup script will reserve a fixed 6 GB of space regardless of sd card
-#     size.
-#   * CAM_SIZE should generally be somewhat small, around 16G to 32G, unless you
+#   * The setup script will keep a fixed 6 GB of space in reserve regardless of
+#     sd card size.
+#   * CAM_SIZE should generally be somewhat small, 30G is recommended, unless you
 #     plan on accumulating a lot of footage between archive operations, in which
 #     case you should increase the value (an hour of recordings, or 6 Sentry
 #     events, is about 7-9 GB of data).
-#   * As of Tesla software version 2020.48.10, the car will warn when the
-#     recording drive is smaller than 32 GB, but smaller values will still work.
-#     Tesla appears to use power-of-ten based sizes, presumably because most
-#     storage manufacturers do too.
-#     Because teslausb uses power-of-two sizes, you can specify "30G" for the
-#     size and not get the warning.
 #   * MUSIC_SIZE should be large enough to hold your music library, obviously.
 #   * If no music size is specified, no music drive will be created.
 #   * If no lightshow size is specified, no lightshow drive will be created.
diff --git a/setup/pi/create-backingfiles.sh b/setup/pi/create-backingfiles.sh
index 71a08203..54d5c5a1 100755
--- a/setup/pi/create-backingfiles.sh
+++ b/setup/pi/create-backingfiles.sh
@@ -55,7 +55,7 @@ available_space () {
 }
 
 function calc_size () {
-  local requestedsize="$1"
+  local requestedsize="${!1}"
   local availablesize
   availablesize="$(available_space)"
   if [ "$availablesize" -lt 0 ]
@@ -65,11 +65,27 @@ function calc_size () {
   fi
   if is_percent "$requestedsize"
   then
-    local percent=${requestedsize//%/}
-    requestedsize="$(( availablesize * percent / 100 ))"
-  else
-    requestedsize="$(( $(dehumanize $requestedsize) / 1024 ))"
+    case ${1} in
+      CAM_SIZE)
+        requestedsize=30G
+        ;;
+      MUSIC_SIZE)
+        requestedsize=4G
+        ;;
+      BOOMBOX_SIZE)
+        requestedsize=100M
+        ;;
+      LIGHTSHOW_SIZE)
+        requestedsize=1G
+        ;;
+      *)
+        log_progress "Percentage-based size no longer supported, use fixed size instead." > /dev/stderr
+        exit 1
+        ;;
+    esac
+    log_progress "Percentage-based size no longer supported, using default size of $requestedsize for $1" > /dev/stderr
   fi
+  requestedsize="$(( $(dehumanize $requestedsize) / 1024 ))"
   if [ "$requestedsize" -gt "$availablesize" ]
   then
     requestedsize="$availablesize"
@@ -136,7 +152,7 @@ LIGHTSHOW_DISK_FILE_NAME="$BACKINGFILES_MOUNTPOINT/lightshow_disk.bin"
 BOOMBOX_DISK_FILE_NAME="$BACKINGFILES_MOUNTPOINT/boombox_disk.bin"
 
 # delete existing files, because fallocate doesn't shrink files, and
-# because they interfere with the percentage-of-free-space calculation
+# because they interfere with the free-space calculation
 if [ -e "$CAM_DISK_FILE_NAME" ] || [ -e "$MUSIC_DISK_FILE_NAME" ] || [ -e "$LIGHTSHOW_DISK_FILE_NAME" ] || [ -e "$BOOMBOX_DISK_FILE_NAME" ] || [ -e "$BACKINGFILES_MOUNTPOINT/snapshots" ]
 then
   if [ -t 0 ]
@@ -196,20 +212,24 @@ then
   apt-get -y --force-yes install dosfstools
 fi
 
-CAM_DISK_SIZE="$(calc_size "$CAM_SIZE")"
-MUSIC_DISK_SIZE="$(calc_size "$MUSIC_SIZE")"
-LIGHTSHOW_DISK_SIZE="$(calc_size "$LIGHTSHOW_SIZE")"
-BOOMBOX_DISK_SIZE="$(calc_size "$BOOMBOX_SIZE")"
+CAM_DISK_SIZE="$(calc_size CAM_SIZE)"
+MUSIC_DISK_SIZE="$(calc_size MUSIC_SIZE)"
+LIGHTSHOW_DISK_SIZE="$(calc_size LIGHTSHOW_SIZE)"
+BOOMBOX_DISK_SIZE="$(calc_size BOOMBOX_SIZE)"
+
+if [ "$((CAM_DISK_SIZE+MUSIC_DISK_SIZE+LIGHTSHOW_DISK_SIZE+BOOMBOX_DISK_SIZE))" -gt "$(available_space)" ]
+then
+  echo "Total requested size exceeds available space"
+  # TODO: reduce sizes to fit
+  exit 1
+fi
 
 add_drive "cam" "CAM" "$CAM_DISK_SIZE" "$CAM_DISK_FILE_NAME" "$USE_EXFAT"
 log_progress "created camera backing file"
 
 REMAINING_SPACE="$(available_space)"
 
-if [ "$CAM_SIZE" = "100%" ]
-then
-  MUSIC_DISK_SIZE=0
-elif [ "$MUSIC_DISK_SIZE" -gt "$REMAINING_SPACE" ]
+if [ "$MUSIC_DISK_SIZE" -gt "$REMAINING_SPACE" ]
 then
   MUSIC_DISK_SIZE="$REMAINING_SPACE"
 fi