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.