MEMORY_MAX_TARGET recommendations. Automatic memory management AMM

Hi,
I am looking at increasing the memory in our databases.

select name, value, display_value from v$parameter
where name like '%sga%'
union
select name, value, display_value from v$parameter
where name like '%memory%';

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I want to go to automatic memory management and increase the memory_max_target to 6G.     Inorder to do this I need to increase the tmpfs to greater then 6G. The default is 50% of memory.
tmpfs                   /dev/shm                tmpfs   defaults        0 0

I read I can do the following.
To adjust the shared memory file system size issue the following commands, specifying the required size of shared memory.

# umount tmpfs
# mount -t tmpfs shmfs -o size=6144m /dev/shm
Make the setting permanent by amending the "tmpfs" setting of the "/etc/fstab" file to look like this.

tmpfs                   /dev/shm                tmpfs   size=6144m      0 0

[root@x ~]# df ls -la /dev/shm
df: `ls': No such file or directory
Filesystem           1K-blocks      Used Available Use% Mounted on
tmpfs                  4194304   2208512   1985792  53% /dev/shm
[root@reptstdwhdb01 ~]# df ls -h /dev/shm
df: `ls': No such file or directory
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 4.0G  2.2G  1.9G  53% /dev/shm
[root@x ~]# 

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My questions are is there any standards for setting automatic memory.
I know I have 8G memory I want to assign 6G to memory_max_target
I know if I don't assign more then 6G to tmpfs I will get an error.
I read on http://johnpjeffries.wordpress.com/tag/shmfs/
that the tmpfs should be 20% more then memory_max_target.

I could drop memory_max_target to 5G approx or increase tmpfs to approx 7G.  I need some guidance. Thanks in advance.
sga.JPG
memory.JPG