We recently developed a high speed null polarimeter [ Opt. Express 30 , 18889 ( 2022 ) OPEXFF 1094-4087 10.1364/OE.454193 ] based on passive polarization optics and a high speed wavelength swept laser source, enabling the measurement of linear retardance with 3.1µdeg/Hz resolution within a minimum acquisition time of 10 µs, corresponding to a linear retardation of 8.6×10 −9 λ /Hz. The counterpart of high sensitivity lies in the systematic errors unlike Mueller polarimeters, which can be calibrated but which are much less sensitive. This paper focuses on the accuracy of this null polarimeter and provides hardware and numerical solutions to improve both linear retardance and azimuth measurements. Experiments and theoretical simulations are carried out to demonstrate the relevancy of these solutions.