There is a commercial program called "Flaresim". I'm in the business and don't use it personally so this is just information, not an endorsement. I'm fairly sure that it just crunches numbers and doesn't have any background thinking built in, so as usual GIGO rules and you have to do your own leg work when it comes to understanding the issues.
There are no secret formulae in Flaring and everything you need is already in the public domain. Check publications and papers published by AIChE and API.
Most people ask the vendors to do the calculations for them. The big US vendors are Flaregas and Zink. Both have websites and Worldwide affiliations. If you need any more of my opinion (which could stray away from the objectiveness needed in this forum), drop me an emai through my web page at