Safety relief valves are generally rated at 1/2 the burst pressure of the vessel and/or 2x teh working pressure under typical conditions. If you must extend the HP side, plan for 375 working PSI and you should be fine. Plus, the SRV will back you up in the event of an overpressure. Can you plan for 200#? Sure, but if that thing sits in the sun and hits 110°F you may have a failure.