Try to be propotion with your actual physical memory.
Your SGA size should be about 1/3 of physical memory.

Query first the SGA size:
select * from v$sga;
or
select sum(value) from v$sga;

Remember not to over size your db buffer, your system might
end up using swap file in which case performance suffer.

Try to maintain your indexes first before any adjustments you have to make in db_block_buffers. In this case, you are sure that the hit ratio
you want to achieve is because of db_block_buffers configuration and not indexes issues.