as far as flow rate, you want a happy medium. You want it to be in the chiller long enough to be cooled, but you don't want it getting chilled too much. (it would be like adding cold water directly to your tank) Of course, perhaps you chiller has safeguards against that. A good one probably will. I ball valve is never a bad idea, for maintenace if nothing else. And as far as back preasure on the pump, it depends on how far closed you think you'll have it. How much head height is there going to be between the pump and the top of the tank where the water returns to the tank?