A variety of types for formal determination of one of the main parameters of field emission systems - the emission area was investigated. Using hemisphere on a cylindrical post model built using the COMSOL Multiphysics software package the relationship between known types of emission area estimation was shown. The corresponding values of emission currents were calculated. It was shown that the formal emission area (the ratio of the total current to the current density at the top) provides only ~ 75% of the emission current value. The effect of the anomalous behavior of the effective emission area (intercept of the trend line of current-voltage characteristics) is obtained when the voltage level changes in standard Fowler-Nordheim coordinates. A method for an experimental estimate of the field emission area using the modified Fowler-Nordheim coordinates ln (I/U^(2-η/6)) vs 1/U is proposed. The method is applied to the analysis of field emission data of a multi-tip nanocomposite emitter with carbon nanotubes.