Hello Sub,
About the SAP memory, the "heap" parameters are related to memory that is allocated on demand, and freed when not needed anymore.
Thus, I would remove it from your formula.
Depending on the operating system involved, it could be either "EM/TOTAL_SIZE_MB + SAP Buffers" or "em/initial_size_MB + SAP Buffers".
You should consider not allocating all the memory you have available.
You should keep some memory free for the OS itself, and even more free memory for the dynamic allocations ("heap" parameters).
Regards,
Isaías