Thinking of all the interior art from The Rithmatest leads me to an important question...
Can you draw a perfect circle freehand? With chalk?
Hahaaaa, not a chance. But even a Rithmatist spends much of their young life learning that skill. Some have it easier than others, but it's not something that just comes to people with Rithmatic ability. It's very much a skill.
That being said, I do think the best way to draw a circle would be to lock your arm out and then spin on one foot, effectively turning yourself into a compass.