How do I gmirror slices?

[Coen Bröcker]

Currently ad4 contains 3 partitions:
ad4s1 swap, / and /var
ad4s2 /usr
ad4s3 /home
I wan't to mirror ad4s1 and ad4s2. I want to stripe ad4s3. Below is my
current slice setup.

Offset       Size(ST)        End     Name  PType       Desc  Subtype
Flags

         0         63         62        -     12     unused        0
        63   12578832   12578894    ad4s1      8    freebsd      165
  12578895  147814065  160392959    ad4s2      8    freebsd      165
 160392960  425706435  586099394    ad4s3      8    freebsd      165
 586099395      15309  586114703        -     12     unused        0

Before I continue setting up the mirror I want to make sure I've setup my
slices correctly.
Reading the gmirror man page it states that the gmirror utility uses on-disk
metadata (stored in the provider's last sector) to store all needed
information.
Since I'm no expert in understanding disk structures I'm not sure what the
above means.

In my case I think I should create the folowing slice setup:
my last partition on ad4s1 must end at 12578893 instead of 12578894. The
next partition should begin at 12578895 and end at 160392958 creating the
space to store metadata.

Can someone please confirm?