The situation is even more complicated when one realizes that more than one item can burn. However, under most conditions, such procedures entail a great deal of uncertainty. When one contemplates the uncertainties then associated with estimating the ignition for the third, fourth, etc. item, it becomes clear that the ignition sequence of a roomful of diverse items cannot be predicted with a reasonable degree of confidence. Model development is a difficult, specialist task. What is possible to do on relatively short notice is to organize fire tests.
Fire tests have their limitations, too.
The largest fire that can be conducted indoors in a laboratory, under controlled and instrumented conditions is about 20 megawatts. Physically, this corresponds to one room or a couple of smallish rooms joined together. Fire models are much less restricted in that respect. Thus, the practical solution is to combine fire modeling with fire testing. Normally, the objects, walls, etc. associated with ignition and early fire growth are directly reconstructed in the laboratory by procuring exemplars and creating what is normally termed a sectional full-scale mockup.