How to calculate shared memory size

Hello all,

I want to do a proper calculation of the number of huge pages needed for 
PostgreSQL to start.

For that I need to know how much shared memory will be allocated at 
startup.

Looking at the shared memory calculation in the source (ipci.c), I can 
see that two terms stand out:

   size = add_size(size, BufferShmemSize()); ---> shared_buffers
   size = add_size(size, XLOGShmemSize()); --> 1/32 * shared_buffers, but 
max WAL segment size

However, if I only use those two terms (shared buffers + WAL segment 
size) for my calculation, the result will be off by a lot, especially 
with bigger shared buffers (e.g. 32 GB).

What are any other major terms (in the order of 10 megabytes or more) 
that I need to take into account to get a better result?

Regards,

tamas