One of the key challenges in geometrical quantity metrology is the evaluation of roundness deviation of rotational elements when access to the full profile is limited. In industrial practice, roundness measurements are often limited to partial contours, which can influence the resulting evaluation of roundness deviation. This study investigates the influence of the analysed arc length and its position relative to characteristic features of the profile on the value of roundness deviation determined using the Least Squares Circle (LSCI) method. Experimental investigations were carried out using a form tester on internal circular profiles with three different types of geometric deformation: oval, triangular, and quadrangular. Arc segments ranging from the full circumference of 360° down to 15° were analysed at three characteristic positions with respect to the profile geometry: at the peak, in the valley, and in an intermediate location. The results demonstrate that shortening the analysed arc length leads to a reduction in the evaluated roundness deviation for all investigated profiles. However, the magnitude and character of this effect strongly depend on the geometric form of the profile. With an increasing number of form waves, the decrease in roundness deviation becomes more gradual and the sensitivity to the position of the analysed segment is reduced. The quadrangular profile exhibited the highest stability of results and the smallest differences between measurement positions, whereas the oval profile showed pronounced variability and strong dependence on the segment location. The findings provide experimental confirmation of the authors’ previous simulation-based studies and demonstrate that the LSCI method can be effectively applied to the evaluation of roundness deviation under conditions of limited profile coverage, provided that the analysed arc length is appropriately selected.