How about this approach, the allowable stress for ASME SA 350 LF2 at 750 deg F is 14.8 Ksi (per ASME B&PV Code, Section II, Part D). At 750 deg F, the allowable stress above is controlled by time dependent effects - like creep deformation. If your calculated stresses are at or below this value, you should be fine with plenty of design margin.