Troy the best focal reducer field flattener that works with a variety of focal lengths is this one.
Here
http://www.sciencecenter.net/hutech/.../html/7887.htm
I have used it with an 80ED 100ED Tal 200K and a Skywatcher 150mm diam F5 achro.
It is only 0.85 reduction but the field is flat even with a full frame camera such as my Canon 5DH.
Bert