I have compared four central 30×30 pixel areas in all the available sky master flatfields. A minimum of 3 masters were available in each filter. The selected regions a (blue), b (red), c (pink) and d (green) are shown in the figure. The flatfield used in the figure is the B master for night JD2455827.
For each filter and region, I have checked the difference between the largest and smallest mean value, expressed as a percentage of the smaller value. These (worst case) changes lie in the range 0.03-0.36%. I seems we can count on the error in a master flatfield to be very low!
The best master flatfields seem to be the B and V filter with only a single case of a change above 0.1%.
The worst region of the four is the green region. This is true for all filters. Perhaps these worst case changes in the worst region can be used as an estimate for the error in a master flatfield?