how to calculate checkpoint_segments

i am trying to optimise postgres 8 running on a system.... is there any way to know how wht should be the value of checkpoint_segments...

 

i always keep fync = false since my server is fully reliable....

 

increasing checkpoint_segments degrade the performace while checkpointing as it will have a whole lot of dirty buffers to write.....

 

decreasing it will degrade overall performance as it will keep on doing checkpointing again and again.....

 

is there any parameters using which i can determine the value of checkpoint_segments.... like how much data my application is generating etc etc....

 

thx

Himanshu