diff --git a/pmb/install/_install.py b/pmb/install/_install.py
index 4048cc6a..cc1d1779 100644
--- a/pmb/install/_install.py
+++ b/pmb/install/_install.py
@@ -607,6 +607,23 @@ def sanity_check_ondev_version(args):
                            f" / in the binary packages has version {ver_pkg}.")
 
 
+def get_partition_layout(reserve):
+    """
+    :param reserve: create an empty partition between root and boot (pma#463)
+    :returns: the partition layout, e.g. without reserve and kernel:
+              {"boot": 1, "reserve": None, "root": 2}
+    """
+    ret = {}
+    ret["boot"] = 1
+    ret["reserve"] = None
+    ret["root"] = 2
+
+    if reserve:
+        ret["reserve"] = ret["root"]
+        ret["root"] += 1
+    return ret
+
+
 def install_system_image(args, size_reserve, suffix, step, steps,
                          boot_label="pmOS_boot", root_label="pmOS_root",
                          split=False, sdcard=None):
@@ -625,16 +642,16 @@ def install_system_image(args, size_reserve, suffix, step, steps,
     logging.info(f"*** ({step}/{steps}) PREPARE INSTALL BLOCKDEVICE ***")
     pmb.chroot.shutdown(args, True)
     (size_boot, size_root) = get_subpartitions_size(args, suffix)
+    layout = get_partition_layout(size_reserve)
     if not args.rsync:
         pmb.install.blockdevice.create(args, size_boot, size_root,
                                        size_reserve, split, sdcard)
         if not split:
-            pmb.install.partition(args, size_boot, size_reserve)
+            pmb.install.partition(args, layout, size_boot, size_reserve)
     if not split:
-        root_id = 3 if size_reserve else 2
-        pmb.install.partitions_mount(args, root_id, sdcard)
+        pmb.install.partitions_mount(args, layout, sdcard)
 
-    pmb.install.format(args, size_reserve, boot_label, root_label, sdcard)
+    pmb.install.format(args, layout, boot_label, root_label, sdcard)
 
     # Just copy all the files
     logging.info(f"*** ({step + 1}/{steps}) FILL INSTALL BLOCKDEVICE ***")
diff --git a/pmb/install/format.py b/pmb/install/format.py
index d9bdbaae..a85179dc 100644
--- a/pmb/install/format.py
+++ b/pmb/install/format.py
@@ -125,14 +125,14 @@ def format_and_mount_root(args, device, root_label, sdcard):
     pmb.chroot.root(args, ["mount", device, mountpoint])
 
 
-def format(args, size_reserve, boot_label, root_label, sdcard):
+def format(args, layout, boot_label, root_label, sdcard):
     """
-    :param size_reserve: empty partition between root and boot in MiB (pma#463)
+    :param layout: partition layout from get_partition_layout()
     :param boot_label: label of the boot partition (e.g. "pmOS_boot")
     :param root_label: label of the root partition (e.g. "pmOS_root")
     :param sdcard: path to sdcard device (e.g. /dev/mmcblk0) or None
     """
-    root_dev = "/dev/installp3" if size_reserve else "/dev/installp2"
+    root_dev = f"/dev/installp{layout['root']}"
 
     if args.full_disk_encryption:
         format_luks_root(args, root_dev)
diff --git a/pmb/install/partition.py b/pmb/install/partition.py
index cd2808b7..02ac5686 100644
--- a/pmb/install/partition.py
+++ b/pmb/install/partition.py
@@ -8,10 +8,10 @@ import pmb.config
 import pmb.install.losetup
 
 
-def partitions_mount(args, root_id, sdcard):
+def partitions_mount(args, layout, sdcard):
     """
     Mount blockdevices of partitions inside native chroot
-    :param root_id: root partition id
+    :param layout: partition layout from get_partition_layout()
     :param sdcard: path to sdcard device (e.g. /dev/mmcblk0) or None
     """
     prefix = sdcard
@@ -37,13 +37,13 @@ def partitions_mount(args, root_id, sdcard):
                            prefix + " to be located at " + prefix +
                            "1 or " + prefix + "p1!")
 
-    for i in [1, root_id]:
+    for i in [1, layout["root"]]:
         source = prefix + partition_prefix + str(i)
         target = args.work + "/chroot_native/dev/installp" + str(i)
         pmb.helpers.mount.bind_file(args, source, target)
 
 
-def partition(args, size_boot, size_reserve):
+def partition(args, layout, size_boot, size_reserve):
     """
     Partition /dev/install and create /dev/install{p1,p2,p3}:
     * /dev/installp1: boot
@@ -53,6 +53,7 @@ def partition(args, size_boot, size_reserve):
     When adjusting this function, make sure to also adjust
     ondev-prepare-internal-storage.sh in postmarketos-ondev.git!
 
+    :param layout: partition layout from get_partition_layout()
     :param size_boot: size of the boot partition in MiB
     :param size_reserve: empty partition between root and boot in MiB (pma#463)
     """
@@ -84,7 +85,7 @@ def partition(args, size_boot, size_reserve):
 
     commands += [
         ["mkpart", "primary", mb_root_start, "100%"],
-        ["set", "1", "boot", "on"]
+        ["set", str(layout["boot"]), "boot", "on"]
     ]
 
     for command in commands: