shared memory 651, freebsd 2.2.7

How much shared memory do you need?

options         SHMMNI=96               # number of shared memory identifiers default is 32
options         SHMMAXPGS=20480         # number of pages a shgment can have and the total number of pages? default
1024                                      # max seg size is 25meg
 
options         SHMSEG=64               # number of segments per process
options         SEMMNI=40               # number of semaphore identifiers
options         SEMMNS=240              # number of semaphores in the system
options         MSGSEG=4096             # max number of message segments is this

With Postgres set for 512 backends, I can't start it.
I can run with 128, but at 10240 have been running out during large transactions.
I am hoping I can get it at 20480.