Ted has some good technical comments on working with the Mark IV images in his previous message. I ramble a little in my reply, but I think there are some fine points worth some review or consensus. I offer a summary of them at the end of the message.
Flats remove gradients by design.
Here is where I disagree. Let me explain. It therefore is not corrected by flat-fielding. Meanwhile he has the best statement I've seen yet to describe the issue, in terms of what someone can measure from the images given. I think it would be difficult to seperate seeing variations and the usual stellar variations unless the effect noted above is pretty strong.