How precisely can the difference method determine the pi NN coupling constant?

The Coulomb-like backward peak of the np scattering differential cross sect ion is due to one-pion exchange. Extrapolation to the pion pole of precise data should allow to obtain the value of the charged pi NN coupling constan t. This was classically attempted by the use of a smooth physical function, the Chew function, built from the cross section. To improve the accuracy o f such an extrapolation one has introduced a difference method. It consists of extrapolating the difference between the Chew function based on experim ental data and that built from a model where the pi NN coupling is exactly known. Here we cross-check to which precision this novel extrapolation meth od can work by applying it to differences between models and between data a nd models. With good reference models and for the 162 MeV np Uppsala single energy precise data which has a normalisation error of 2.3%, the value of the charged pi NN coupling constant is obtained with an accuracy close to 1 .8%.