In the calibration and alignment of an X-ray reflectometry ("XRR") system for measuring thin films, an approach is presented for accurately determining C₀ for each sample placement and for finding the incident X-ray intensity corresponding to each pixel of a detector array and thus permitting an amplitude calibration of the reflectometer system. Another approach involves aligning an angle-resolved X-ray reflectometer using a focusing optic, such as a Johansson crystal. Another approach relates to validating the focusing optic. Another approach relates to the alignment of the focusing optic with the X-ray source. Another approach concerns the correction of measurements errors caused by the tilt or slope of the sample. Yet another approach concerns the calibration of the vertical position of the sample.