Back pressure reduces set pressure on a one to one basis, i.e., a valve set at 100 psig subjected to a back pressure at the outlet of 10 psig will not actuate until system pressure reaches 110 psig.
Back pressure drastically reduces capacity. Typically backpressure of 10% of set pressure will decrease capacity by 50%. Specific capacity reduction should be determined by the user on a case by case basis by flow testing. Back pressure in excess of 10% of set pressure is not recommended.