multiple pools with one virtual server

So how about this:

 
 when CLIENT_ACCEPTED { 
 if { ([active_members pool_A] < 1) and ([active_members pool_B] >= 1) } { 
 pool pool_B 
 } 
 if { ([active_members pool_B] < 1) and ([active_members pool_A] >= 1) } { 
 pool pool_A 
 } 
 } 
 

Is there a better way to select the event than CLIENT_ACCEPTED? I'd rather not trigger the rule too often. Can I use LB_FAILED?

So, the way to use this iRule is:

- select the local pool as the default pool for the virtual server

- add this iRule to the resources used by the virtual server.

Correct?