Are you hitting the right back focus requirements for the WO field flattener with your DSLR? WO recommend the following from their web site.
FLT 98 - 76.8 mm
FLT 110 - 73.5 mm
FLT 132 - 71.5 mm
GT 102 - 76 mm
M90 - 77.3mm
So you need to ensure that your Megrez 90 has a space of 77.3mm between the rear element of the flattner and the CMOS chip of the camera for the corrector to work optimally. This in turn will alter the imaging train's focal point. It sounds like you need more space if you are racking the focuser all the way in.
http://www.williamoptics.com/accesso...BFL122~142.pdf