From every measured signal, a corresponding frequency spectrum can be found, and by using Eq. (1), the velocity can be calculated with a reference velocity based on Eq. (2). The uncertainty of the radius $\Delta R$ is defined as the size of the laser spot on the paper; all other errors are limited by the instrumentation. The relative error on the reference velocity can be calculated by differentiating Eq. (2) with the angle variation between the laser and the disc giving the most significant uncertainty. The experimental uncertainty of the velocity, based on the uncertainties in the experimentally measured quantities, was calculated according to $\delta Vlaser=(\delta R)2+(\delta \nu )2+(\delta \alpha )2$, where $\delta R=\Delta R/R$, $\delta \nu =\Delta \nu /\nu $, and $\delta \alpha =tan(\alpha )\Delta \alpha $. The calculated dependency on the velocity uncertainty of the setup from the angle is shown in Fig. 5 with a solid black line with a 5% level of the accuracy by the dashed line. For calculations, it was assumed that the angle could be measured with a 0.5 deg precision. The rotational velocity $\omega $ was defined with an accuracy of 1%. The size of the laser spot on the paper depends on the angle $\alpha $ as well being proportional to the cosine of this angle, leading to an increasing relative error of the measurements at small, $<10\u2009\u2009deg$, angles due to a broadening of the frequency spectrum. Speckle modulation increases dramatically at angles $>80\u2009\u2009deg$, which also results in frequency spectrum broadening.^{19}^{,}^{35} Considering the trend of the calculated experimental uncertainties, presented in Fig. 5 by a black solid line, the optimal range of angles from an accuracy point of view is between 13 and 77 deg, which gives an error of $<5%$, and between 25 and 63 deg, which gives an error $<3%$.