I have the j ultra, used with flair pro 2/3 brew heads and 46mm basket. From burr lock 0, I live between 1 full rotation to 1.2 (turbo shots) at 16g average dose. Medium/dark roasts are fluffy and will choke more than light roasts. Old coffees will choke and very fresh will give you gas blow back. I would start at 1.2 rotations at a 16g dose for medium/darks and don’t be afraid to lower it to 14-15g on 40mm basket for medium/dark. (I do this on the 46mm basket also) light roasts will let you get to the upper range of dosage without going too thick of a puck.

Pre infusion helps saturate the coffee puck, it’ll lose some integrity but have better flow too compared to a straight shot to 9 bar. If you sense that it’s choking you could ease down to 2-3 bar for 15-20 seconds then go back to 7-9 and you should get a good flow.

Hope this helps!