Confused about the buffer pool size

Hello All:
In the code (costsize.c), I see that effective_cache_size is set to DEFAULT_EFFECTIVE_CACHE_SIZE.
This is defined as follows in cost.h
#define DEFAULT_EFFECTIVE_CACHE_SIZE 16384
But when I say 
show shared_buffers in psql I get,
shared_buffers ---------------- 28MB
In postgresql.conf file, the following lines appear
shared_buffers = 28MB                   # min 128kB           # (change requires restart)#temp_buffers = 8MB                     # min 800kB

So I am assuming that the buffer pool size is 28MB = 28 * 128 = 3584 8K pages.
So should effective_cache_size be set to 3584 rather than the 16384?
Thanks,
MMK.