Think I'm using a 4.1 or 4. It's personal. You can test for it if you have a few to play with but resist the temptation to go too tight. If under pressure it can exaggerate hold movement on the bull and also the eyes can have trouble discerning a clean even gap between the ring and the bull.
Also worth playing with rear iris size as well, which can vary with lighting and preference.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep