Typically, the X-ray beam has the highest intensity in the center and its shape can be fitted with a Gaussian. Thus, the beam size is characterized by the full width at half maximum (FWHM) in vertical and horizontal directions, which in our case is 22x10 μm. The intensity profile of the reflected beam on the detector, such as the one in Figure (black curve), can also be fitted with a Gaussian. Here, we get an FWHM of ~70 pixels. Using a pixel size of 55 μm, we derive l = 3.85 mm. With L = 488.4 mm and αi = 2.05°, and applying Equation 2 and 3, we find R = 157 and S = 615 μm. Consequently, a pixel of the detector corresponds to the distance of ~9 μm at the sample surface. This ratio is applied to the top x-axis of Figure and 4a, thus helping to correlate the intensity drops with the flakes' movement on the liquid Cu surface. At these conditions, the samples with larger R can be used as well, e. g. up to R = 500 mm, which would result in a magnification factor of 2. The curvature R defined via the spread l is the local curvature of the surface and is not necessarily constant for the entire drop. We also note that we do not observe a change in the R between the bare and fully covered states within a single measurement (30 min). It is also possible to deduce the coverage ratio under the beam by dividing the data in Figure into two components: covered and non-covered areas, the resulting time-evolution is shown in Figure (blue).