It doesn't look like anyone has gotten specific with respect to references, so I would suggest starting with API Recommended Practices 520 and 521. In addition to the cautions of other folks who have responded, these references are the most direct route to a starting methodology.
Can anyone direct me to a definition for PSV rated flow? A free definition, that is (other than ASME PTC 25). I have scoured through API 520 and 521 and find many references to rated flow, but haven't yet come across an exact definition.