A 45 degree single flare on copper or aluminium tube fails in a few repeatable ways. The cut was not square, the burr was left in, the nut went on too late, the tube stuck out too far, or the seat was cracked and then tightened anyway. This is the order I use before the joint is closed.
Cut it square
Use a pipe cutter, not a saw. Take a small turn at a time so the tube does not oval. A sawn end rolls that error into the seating face.
Ream with the tube pointing down
Ream the inside edge until the burr is gone. Keep the open end down so the swarf falls out of the tube, not into the system.
Slide the nut on first
Put the flare nut on the tube before the tube goes into the bar. After the flare is formed, the nut will not pass over it.
Leave about 1-2 mm proud of the block
Seat the tube in the matching hole. For a single 45 degree flare, about 1-2 mm should stand above the face of the bar. Too little and the flare is thin. Too much and the cone splits the copper.
Stop when the clutch clicks
Line the eccentric yoke up, clamp it, and turn the handle until the clutch clicks and spins free. That click is the stop. Further turns do not improve the seat.
Look at the face before you tighten
The seating surface should be smooth and even, with no crack from the lip. If it is cracked, cut that end off and flare it again. Tightening a cracked flare only hides the weep until the system is under pressure.
These steps are the ones I follow with the eccentric set written up at https://flaritool.co.uk
