Did you know, the allowable total backpressure for a conventional valve can be larger than 10% of the set pressure? According to API 520-1 §5.3.3.1.3:
"In a conventional PRV application, when the allowable overpressure is 10%, the built-up backpressure should not exceed 10% of the set pressure. A higher maximum allowable built-up backpressure may be used for allowable overpressures greater than 10 % provided the built-up backpressure does not exceed the allowable overpressure."