Or maybe he goes to the room, gets the Mirror, and looking into the Mirror to correct himself, draws the circle just right.

(Since he does want to be able to make a Philosopher's stone, not just to get one - he wants 'mass-produced immortality'. And he had suggeted to Hermione that magical objects could be used to draw objects more precisely (only they were discussing a different object). And he had already used a supposedly-significant stone in battle in a least-magic-demanding way. And we haven't seen even in Rowling's world that the Mirror can only show 'real-sized things', so it can potentially magnify them.)