A self-calibration method was presented for a 3D Ultrasound Tomography System (USCT) in IEEE Ultrasonics Symposium (IUS) 2015 . The method sequentially calibrates a complex USCT system with 2041 transducers based on time-of-flight (TOF) measurements. A direct evaluation of the calibration result was not possible due to unknown ground truth. In this work we present a method to estimate the upper boundary for the calibration accuracy. Evaluation with experiment data shows an estimated upper boundary of the mean error of 0.11 mm, which is smaller than the required accuracy of /4 = 0.15mm for high quality image reconstruction .