Thank you for clarifying that Greg - it makes perfect sense now. I stand corrected. It’s a happy day - I learnt something new
I think what reinforced my mistake was trying to fine-tune a spacing for my Riccardi reducer. I am getting better results when I reduced recommended by APM for my telescope’s focal length physical distance between the Riccardi and the camera by about 1.5mm, and when I added a 1mm spacer correction was clearly worse.