Query related to SGA & PGA allocation

 We have got a server with 64 GB  RAM (Linux RHEL 5.2).
This is the first server with 64GB RAM in our environment.

So far we were having the server with 32GB as the max RAM size.

In general, we use to set the values with below formula,
pga_aggregate_target=<value> #20% of the RAM size
sga_target=<value> #40% of the RAM size
shared_pool_size=<value> #20% on the SGA_TARGET value
db_cache_size=<value> #30% on the SGA_TARGET value

There will not be any other process run in the DB server other than Rman backup & export backup once in a day. So, this formula is formed to ensure that the maximum possible memory will be allocated for SGA & PGA.