my personal opinion is that 120Mo, for the two servers is the highest you can figure out for each db, so you ok.
You must pay attention to not outrun the real memory of the machine, so to avoid the disk swaping.
Anyways, you can issue:
Select bytes/1024/1024 from v$sgastat where POOL='FREE MEMORY';
If this value is consistently high over time, another area of SGA needs memory, you may want to decrease SHARED_POOL_SIZE.