My middle school maths teacher (awesome guy, also had a mustache - mid 90s) taught us how to draw (large) circles on blackboards easily: Start from the bottom

My middle school maths teacher (awesome guy, also had a mustache - mid 90s) taught us how to draw (large) circles on blackboards easily: Start from the bottom and go counter-clockwise full circle having your arm fully extended. After the first couple of tries, when you build up confidence through the results, the circles get very good. The trick is not moving slowly, but doing it in one go. The 3'/two ark circles he would do the same way, but using the elbow as the central/pivot point instead of the shoulder.