I adjusted the value for the heat of combustion of ammonia  it should be about 382.6 kJ/mol.  The heats of formation are -46,100 kJ/mol and -285,830 kJ/mol for ammonia and liquid water, respectively (Smith, Abbot, and Van Ness.  Introduction to Chemical Engineering Thermodynamics, 5th edition. 1996.  Table C.4, pg. 641).  Nitrogen and oxygen have 0 heats of formation, by definition.  The standard heat of combustion at 298 K will then be -285,830*1.5 + 46,100 = 382,600, given a reaction equation of 3/4 O2 + NH3 -> 1/2 N2 + 3/2 H2O.