I have the same camera and scope. I found that, referring to the graph in this thread for the spacings
https://www.cloudynights.com/topic/4...ith-stf-8300m/
a 0.75x reduction with the CCDT67 worked ok in terms of star shapes with the ASI1600 and the RC8 (which is about 60mm sensor to rear flange of CCDT67).
Any bigger reduction than that and the stars at the edges started to look a bit dodgy.
Worth noting that at 0.75x with that camera and scope, the resolution is about 0.65 arcseconds per pixel. You don't want to go much less of reduction than 0.75 then, as then you'll be risking oversampling.
Paul