How familiar are you with ASME VIII Div 1? I assume thats the Code you're designing to. If you dont understand or know the Code, the use of software (regardless how good it is) is useless, and more important, dangerous.
Start reading ASME VIII Div 1 (and buy a copy to own), and pay attention to UG-28. If there's someone in your company to mentor you, ask him to train you over the years. If there's no one in your company to mentor you, hire a consultant to do the work. If your boss doesnt have that money, tell him you're not quailified for this work and you cant do it.
I suspect the formula may be derived from UG-28(c)(1) step 6, but I'd need a copy of PVElite to verify.