There are two methods I use frequently the more detailed with all the formula is IEEE 515 I believe revision 1997 is the latest. This is very good. The easier method is a heat transfer program written by Nelson heaters at Nelsonheaters.com The program is Renegade and is free. I have don't thousands of these type calculations using Renegae. If you buy the heater cables from Nelson they will assist you in the calculation for free.