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X-ray cone-beam computed tomography (CBCT) imaging based on the single circular orbit scan mode has become more and more popular because of its high imaging speed. Generally the X-ray source and detectors are stationary and the scanned object rotates step by step over 360° in this kind of system. In order to improve the imaging efficiency, the step rotation is sometimes replaced by the continuous rotation. For this continuous rotation, if there is an error about the number of projection images over 360° caused by the uncertainty of synchronization between the detector and the mechanical sub-system, the angular increment for the CT reconstruction may not be known exactly. The error will lead to serious artifacts in the CT images. This paper describes two possible methods determining the angular increment. Both of them are based on the statistical similarity among the projection images. They determined the angular increment by calculating the correlation coefficients and the standard deviation of the difference image between the image at 0° and the images at other angular positions. These two methods can verify each other. We performed numerical studies and experiments to demonstrate and validate the proposed approaches. Results in these studies and experiments confirm that the proposed two methods can determine the right projection number and the angular increment. The proposed approaches may find applications in CBCT system with continuous single circular orbit scan mode.