Thanks Paul. I've been doing some more reading since making this post and I gather from a chat over at Cloudy Nights that most of the .6x focal reducers are more for SCTs and Maks. They introduce coma(?) at the edges on refractors.
I actually have a Mogg .66 reducer that I've never used for
reasons posted here. Might have to make a separate post asking how to repair it so I can test it out.