In continuous flow reactor analyses, the experimentally obtained molar flow rates of unreacted reactants and formed products are called useful if the corresponding atom balances are closed, i. e., within 90-110%. This manuscript (1) shows that even for the validity condition of a balance between 95% and 105%, deviations for conversion and selectivity can be very pronounced and (2) offers, likewise, a very simple a posteriori calculation procedure to address these deviations. To validate the proposed procedure, called 'CLOBAL'-after 'closing the balances', an example from butane oxidative dehydrogenation (ODH) is taken and it is clearly shown that the CLOBAL method gives values for conversion and selectivity, closest to the 'real' values, compared to the conventional methods, i. e., without correction. As an example of batch reactor operation, the aldol condensation towards jasmine aldehyde is taken to show that the application of the presented CLOBAL procedure gives corrected concentrations values which result in more accurate kinetic parameter estimates with a lower residual sum of squares.