I presume you meant 400 microns, not 400 mm? 400mm is almost 16 inches, which would be 'bent' indeed. Apple's tolerance for bending is 400 microns, or about the thickness of four sheets of paper.
Take the iPad, put it on a very flat surface ("flat" in this case is a flat as you think you need, you can use an electron tunneling microscope to arrange the atoms, if it's that critical to you, or you can just use what an average person would define as "flat.") and see if you can put a stack of 4 sheets of paper under it at the maximum gap. Now the thickness of the paper is variable, too, so you may need to break out that ETM again to ensure that the paper is of proper thickness and fits within "standard," whatever that may mean. And by "stack" you need to decide how much air you want to permit between the sheets of paper. Maybe use a vacuum chamber to do the stacking, keep the paper in the vacuum until the pages adhere tightly to one another to compress as much as you can. You'll have to allow some tolerance in the thickness because when you take the paper out of the vacuum to slip it under the iPad there will be some air slipping between the pages. I don't know your personal level of acceptable air intrusion, but it's all up to you. And also consider that the temperature of the iPad, the flat surface and the paper will cause the thickness of the paper to change, the width of the gap to change and maybe even warp the flat surface. There are a lot of variables for you to consider in assessing the "flatness" of that iPad. And the amount of the bend will probably change as the unit warms up in use or cools down when idle, and the battery may swell a bit as it ages to change that bend, so you will need to repeat this process every few minutes to be sure. And you may need to keep meticulous records just in case the flatness is cyclical, to see what may be contributing to the bending at certain times of day.
Or you can just turn it on, give it a test to see if it works well and if it does, then put it in a protective case and forget the bend. It's going to change, anyway.
Your call.