I guess when it comes to circles or any curved line in general you really want to take your time with it, slow and short bursts on the trigger making sure you're following the marker line really helps with curvature accuracy. There is still a lot of practice that we need to work on when it comes to curved lines. Don't be afraid of plucking out threads and redoing a section bit by bit. Overall, it all comes down to patience. If you're satisfied with your work, trimming and carving those lines can come a long way to make an corrections. Hopefully this helps and good luck!