Are you concerned they are not flat? The method I use is to take two sets of flats one with the light source or camera rotated 90 degrees with respect to the other. Use one set to calibrate the other as if one set were actually lights. The resulting image should be entirely smooth within 1 or 2%. If it is not then your light source is not flat or there is some other aspect of your system producing shadows or reflections in the flats that are not present in the lights.
