Tweak image sizes for BD media

We used to have some arbitrary hard-coded sizes; tweak those to allow
for ~4% losses for spare sectors if people format media.

debian/changelog

@@ -6,6 +6,7 @@ debian-cd (3.2.2) UNRELEASED; urgency=medium
   * Add a way to exclude named firmware packages from media, so we can
     control which packages are included - see
     tasks/$codename/firmware-exclude. Closes: #1032071
+  * Tweak sizes slightly for BD media.
 
  -- Steve McIntyre <93sam@debian.org>  Mon, 12 Jun 2023 08:36:50 +0100
 

tools/make_disc_trees.pl

@@ -768,12 +768,12 @@ sub get_disc_size {
         $maxdiskblocks = int(8500 * $MB / $blocksize) - $reserved;
         $diskdesc = "8.5GB DVD";
     } elsif ($chosen_disk eq "BD") {
-		# Useable capacity, found by checking some disks
-        $maxdiskblocks = 11230000 - $reserved;
+	# Leave some space for potential formatting/spares on BD
+	$maxdiskblocks = int(24000 * $MB / $blocksize) - $reserved;
         $diskdesc = "25GB BD";
     } elsif ($chosen_disk eq "DLBD") {
-		# Useable capacity, found by checking some disks
-        $maxdiskblocks = 23652352 - $reserved;
+	# Leave some space for potential formatting/spares on BD
+	$maxdiskblocks = int(48000 * $MB / $blocksize) - $reserved;
         $diskdesc = "50GB DLBD";
     } elsif ($chosen_disk =~ /STICK(\d+)GB/) {
         $maxdiskblocks = int($1 * $GB / $blocksize) - $reserved;