upper threshold = 47000 ADU: Due to the inhomogenous illumination of the UVB imaging flats, the most illuminated pixel regions begin to saturate (counts = 65535 ADU) for frames with median counts > 47000 ADU.
lower threshold = 40000 ADU: Sampling smaller flux ranges means covering the linear part of the detector response curve only. The non-linearity expressed as a fraction at the reference flux level of 50000 ADU should be based on a flux range exceeding the 40 000 ADU. Otherwise the linearity QC parameter becomes more and more a function of the flux range.