Angular misalignment of a sample in a single crystal neutron interferometer leads to systematic errors of the effective sample thickness and in this way to errors in the determination of the coherent scattering length. The misalignment can be determined and the errors can be corrected by a second measurement at a different angular sample position. Furthermore, a method has been developed which allows supervision of the wavelength during the measurements. These two techniques were tested by determining the scattering length of copper. A value of bc = 7.66(4) fm was obtained which is in excellent agreement with previous measurements.