Hi Andrew,
Are the low and high ADU flats conventional imaging or spectroscopic flats? The reason I ask is my individual ALPY spectroscopic flats for example can typically range from a few hundred to a few tens of thousand ADU even within the flat due to the instrument response and lamp spectrum. If they are conventional imaging flats I could perhaps take a few of these to supplement my test data set of MILES stars posted on here for you to test.
Cheers
Robin