Shared Buffers

When starting postmaster, you can add the -B option to indicate the amount
of shared buffers you want postgres to use.  You can also specify the value
in postgresql.conf.

I read in the documentation that 1 shared buffer is 8192 bytes.   Somone
posted a while back that it was most optimal to let postgres have about 1/4
of the memory on a dedicated machine.

So, I have 512 megs on a dedicated machine.  I divide that by 4 and then
times the number of bytes in a meg and divide by 8192 and I get 16384.
By the 1/4 standard, it seems I should set "-B 16384" at postgres startup.

Postgres defaults to 64 Shared Buffers.  That is a big difference from
16384.  Is there any reason to my logic here or am I way off?

Can anyone supply some more information or maybe suggest the most
optimum way to use Shared Buffers.  Is the 1/4 standard good or should
I use even more.. or much less?

Thanks in advance.
Dennis