The work focuses on Periodically Poled Lithium Niobate crystals grown by the off-center Czochralski technique and exhibiting an appreciable Second Harmonic Generation efficiency. Several samples have been characterized through experimental measurements of domains periods in order to find out the real period values and drifts of the border domains positions. By means of the Nonlinear Bidirectional Beam Propagation Method (NL Bi-BPM), the governing equations describing the interaction between the wave at fundamental frequency and the wave at second harmonic have been integrated and the theoretical SHG efficiency was numerically determined. In order to take into account the border domain effects, the normalized efficiency was used to rescale the calculated value furnishing the real SHG efficiency. As an interesting result an efficiency of 0.5% in forward QPM SHG has been predicted for a reference structure of 0.5mm length.