cache_mem parameter

Peeps,
        Hope you can help. I have a Sparc US1, 512MB of RAM, 10GB of disk
for our web cache.
        What size should the cache_mem parameter be set to, given that the
machine does nothing else apart from be the web cache? Should this
cache_mem parameter be close to the total amount of memory available (minus
amounts for the DNS servers and ftpget processes) ?
        At the moment, it's set to 164, which to me appears to be low, but I
can't find out any more information about how to calculate a good value for
it.
        Any help appreciated and any more information if needed can be
supplied.

Rich.

-- 
..Blue         O            "Smoke me a kipper,
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         ...../    Dash..    Email: rich@ops.netcom.net.uk