Calculation of harmonic emission limits is established based on a number of assumptions. After the customer installation is connected to the network, the actual conditions, that might be different from these assumptions, should be considered in the emission assessment to ensure an efficient utilisation of the harmonic hosting capacity of the power system. One of these assumptions is the diversity between harmonic emission of customer installations and the existing background distortion due to their random behaviour. The aim of this paper is to extend the emission assessment method recommended by IEC (e.g. IEC TR 61000-3-14) to take the actual level of cancellation into account. Therefore, the “difference index” is introduced and examined based on field measurements. In addition, a proposal for the permissible level of customer emission considering the actual level of cancellation is provided.