FERS Order

I represent a spouse going through a divorce.  His wife has a FERS benefit and he is getting half.  He wants a survivor annuity (he insists he will live longer than his wife - his call not mine).

To make sure he gets the maximum should I give him 1/2 of the gross annuity, self-only annuity, or net annuity?  It seems to me the gross annuity is the self-only annuity minus the cost of the survivor annuity, so those two choices are effectively the same here.  I'm not sure though if the net annuity would be larger.  I don't think it is, but I'm not sure.  Any thoughts?  Does it depend on other factors?