IEEE provides an EXCEL spreadsheet for use in calculating arc flash. Although given the best intentions, I would be leary about using something else without having specific written documentation that the spreadsheet has been properly verified and validated for accuracy.
There are many variables to enter into the equation so one must be careful that the spreadsheet pertains to their system i.e. grounding, voltage, arc in open air vs a box, conductor gap... the best bet is to make your own.
The equations are given in the back of NFPA 70E 2004 annex D. http://www.arcadvisor.com/arcflash/ieee1584.html has an online calculator.
I bought videos and workbooks from www.brainfiller.com to self teach myself how to conduct an arcflash analysis. A short circuit and a coordination study need to be performed first, to collect the data to be enterd into the equation or calculator.
Good Luck