Oooh. I thought of another way to do it. You could fold a large piece of paper (bigger than 41 inches) in quarters, then place the center point on the floor under a hinged door so that the paper is directly under the door, with the center aligned to the hinge point of the door. Tape it to the floor, then tape a marker 20.5 inches out on the door and gently open and close the door to scribe the arc. Cut your paper along that line while folded, and theoretically, you will get a nice circle.
I would probably do this method and find that it does not work as well as the more conventional tack and string way, and so resort to that anyhow.
But it's fun to think up different ways. Ways that would make my husband and sons shake their heads in mystification. . .
RST