To j2bprometheus:
1. The expression is empirical, quoted from M.G.Zabetakis,"Fire and Explosion Hazards at Temperature and Pressure Extremes," AIChE-Inst. Chem. Engr. Symp. Ser. 2, Chem. Engr. Extreme Cond. Proc. Symp., 1965, pp 99-104.
2. Pressure has little effect on the LFL except at very low pressures (<50 mm Hg abs.), where flames do not propagate.
Besides, here are several threads that dealt with similar subjects:
Thread798-33615
Thread798-50854
Thread798-89295
Thread610-33619
Thread610-40973
Thread135-70447
No doubt there are more threads dealing with ammonia and hydrogen flammability limits.