Just about any book on electric power systems will have some sort of methodology outlined for calculation. As ijl mentioned, Grainger and Stevenson's book is good, but you can also look at Glover and Sarma's Power System Analysis & Design, the Westinghouse T&D book, EPRI's materials (one is Transmission Lines, 345 kV and Above), etc. You could also check with IEEE -- I'm sure that multiple papers on the subject have been published in their transactions over the years.
I am not aware of any online references, but I'm quite certain there is some sort of information about this topic on the web somewhere. Spend some quality time with google and I'm sure you'll come up with something.