you'll need to know your camera too. I'd then go along to one of the FOV calculators, such as http://astronomy.tools/calculators/field_of_view/ and plug all the details in.
Your smallest FOV will obviously be without your focal reducer, so get all the details in, and then find 20% of that FOV. Your largest will be with the Focal reducer, so there is your second parameter.
Dial them in, and go out for the day - it can be a long process to download them all, but thankfully you only need to do it once!