Despite many fundamental objections, TNT-equivalency methods are widely used for simple vapour cloud explosion blast modelling. Presently, however, the TNO Multi-Energy method is increasingly accepted as a more reasonable alternative. Applying the Multi-Energy method, blast can be modelled by the specification of an equivalent fuel-air charge, which is characterized by two parameters: a charge size and a strength. Subsequently, the blast wave parameters at any distance from the charge can be read from fuel-air blast charts. So far, the TNO user recommendations to determine charge size and strength were based on simple assumptions which are believed to be safe and conservative. If such a safe and conservative approach leads to unacceptably high blast overpressures, the user may determine a more accurate value for the charge strength by seeking correlation with available experimental data. To allow a user simple and quick access to the experimental data, a parameter combination has been devised. This parameter combination characterizes the initial and boundary conditions that largely determine the strength of a gas explosion. The parameter combination was correlated with explosion overpressures available from experimental data sets. The result is an instant data base in the form of graphs. The graphs allow a user to estimate the charge strength on the basis of experimental data.