I always used to use the rule of 1/4 inside the wing tip as a good DOWNWIND spacing. it seemed to work. whether it was a C10, AP38, PA28.. whatever. now I have moved onto aircraft that have MORE performance, I find that tghe runway ON the wingtip seems to work better. I would presuem that aircraft with MORE performance (E120 etc) that are now having to fly a 1500' cct would probably be wider again!!!
I was always taught form ab-initio, that wdownwind spacing was JUST enufff to let me GLIDE to a place wher could land on the runway!!!(which was NORMALLY 1/3 inside the wing tip!_)