How to set the DB_CACHE_SIZE to a fixed value

Hi...I have set the DB_CACHE_SIZE(1g), but when I run an ADDM report when the customer is complaining of slowness, the report shows says I have an Undersized Buffer Cache and the value is always different(for example, one run says the value is 672m and that I should increase it to 704m, then another run says it's 740m and should be increased to 810m)  .  The value I set the DB_CACHE_SIZE to is always larger than what the report says it's currently set to.  The customer has sql statement issues, but they are always focusing on the undersized buffer cache finding,