thou this is not the answer to your question, I would like to suggest to use ASMM by setting the value for SGA_TARGET parameter instead of manually setting the values for different pools. You can see more information on ASMM in one of the links provided by Stecal.

for ex you can set 40% of the physical memory to SGA_MAX_SIZE (20 GB in your case) and based on DB size and requirement you can set SGA_TARGET value (you can start with 8GB and monitor). If the statistics shows the need of more memory then you can dynamically increase SGA_TARGET parameter value.

Thanks,