Hi Shiroi98, Glad you are practicing in front of a mirror, that is excellent! I like to balance my weight evenly between my feet. When I was first learning to play, the teacher I had at the time suggested an L shaped stance, resting back on the left foot. Throughout the years, and many professional teachers, I have concluded that we should be able to stand, just as we normally would, even while playing the violin. There really shouldn’t be any noticeable difference in your posture, other than the violin extending from your shoulder. Hope that helps!
Violin Tutor Pro Teacher