Hello
It does not really make sens to increase cache efficiency targeting a specific ratio...
You can try to increase it but there are limits related to the usage / available memory...
You Oracle memory dispatching is quite Ok for SGA (buffer cache / shared pool) ...
PGA seems too small (OLTP = 20 % of Oracle available memory OLAP = 40 % of Oracle available memory)
What is your Oracle version ?
What is running on that server ?
Is there is an SAP server running on it, what is roughly its memory usage ?
How many workprocess, users ?
If some of the memory is not used you can increase the SGA size and then increase accordingly buffer cache (DB_Cache_Size). This should increase you DB cache efficiency.
You are not supposed to define shared_pool_reserved_size (get that advice from a GLC report)
Never forget that SAP buffers (the sap own data buffer cache) are far more efficient than Oracle cache... Oracle has no specific knowledge about the data in its cache.
Tables are buffered at SAP level because it can have a major impact on data access.
Thus check
1) is there free RAM on my server
2) is SAP memory well setup, could it benefit from extra RAM ?
and then you can check if it can be useful to extend Oracle SGA RAM
Best regards