PS. I forgot to add, jtr you are quite right with your interpretation of the CI=0. If the opch is zero then you can keep the airplane in the sky all day, for the crews, maint etc.... are not costing you a penny (not realistic) irrespective of how expensive the gas is. Or if you have normal operating costs but VERY, VERY expensive fuel then you want the cost index close to zero as well to minimise the fuel burn for the sector. being up forever is no fun.
Bye.