A formal investigation describing the failure limit of the spline aided solution technique has been carried out. The influence of spline width on trajectory assigning accuracy at different axial positions is discussed in detail. Additionally this influence has been investigated widely concerning the accuracy of aberration calculations for two different cases. In the first one, spline width was made variable for both trajectory and aberration calculations. The second one analyzed the case in which this width varies only for aberration determination. Results clearly show that spline width is the only parameter that affects calculations. Generally, the technique's accuracy is found to be proportional to the quadratic spline width and hence accurate results can be reached as long as the spline width is small.