I have the same Reducer/Flattner.
Download the trial version of CCD inspector from CCD Ware. It will analyse the image and tell you how flat your image is. It takes the guess work out of it. It gives you a 3D curvature map so you can make the correct adjustments. Otherwise is a frustrating trial and error approach. Remember that you have to refocus after every adjustment.
You may still need to have spacers in between the camera and the flattner to achieve the flattest field possible.
With my STF-8300m, I'm using an Orion flattner an I have a filter wheel and about a 2cm spacer between the corrector and the wheel. I haven't used the flat 4 on this camera but the principle will be the same. You may or may not need the spacer.
Link to CCD Inspector
http://www.ccdware.com/products/ccdinspector/