Refer to API520, Parts 1 and 2 and also API 521; regarding radiation radius from vent tip for various allowable radition levels in BTU/HR. Limit is variable depending on what you wish to protect and how long an escape time you need to allow. Somewhere between 500 to 3000 BTUH is typical. You do not need to regenerate these equations. The more you learn, the less you are certain of.