I know it's an old thread but I want to make sure the reports keep coming... I was unable to get anything close to a flat field with my ES ED80 FCD100 and the ES Field Flattener (55mm model). I tried back focus distances from 45mm to 80mm! (including 0.5 mm increments from 52-58mm) and never got anything even close to acceptable... the image showed 60% of the entire field had significant curvature and the outer 25% had such severe distortion the the stars smeared like an inverse coma. The "best" back focus (still horrible) for me ended being somewhere between 75-80mm despite having a published spec of 55mm. After 5 weeks of trying on clear nights to resolve the issue I gave up and ordered a Hotech SCA FF and instantly had way better images. Ideal back focus proved to be 55mm with the Hotech, just as advertised, and leaves me with mild distortion only in the outer 5% of the image, slightly worse on my left side (maybe 10%) but that I diagnosed as sensor tilt on my camera that I'll have to try and resolve with a tilt plate. I'm pretty new and I don't know how "perfect" I should expect a $1300 scope and $200 field corrector to be (should I truly have a perfect flat field corner to corner?) but I now know at least I have a very nice image cropping a very small amount... the ES FF is a mess for their own ES ED80 scopes and I would avoid them.
I spent literal months in the exact same position as you, same scope and everything. and I ended with the Hotech 2" SCA FF, with about 80mm of spacing (maybe more, I don't really remember, been a while since I measured). I now have a visibly perfect flat field (other than a little tilt as well, which honestly I don't worry about because it is hard to notice after stacking, cropping, and processing, unless you are looking for it). It is one of the hardest and most annoying things I've ever done... Keep trying it sounds like you are almost there!