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Have you tried "Flammability Characteristics of Combustible Gases and Vapors", U.S. Bureau of Mines publ no 627.
It doesn't have everything you could ever wish for but it's an excellent starting point and has a heap of reference documents too.
I have a couple of pdf files with graphs of natural gas LEL/UEL vs temp and pressure if you send me your email address to firstname.lastname@example.org. The graphs, however are no substitute for a clearer understanding which you can get from the full book because the basis isn't just a single easy formula.