Get a stake and some twine. Have one person hold the stake in the middle of the circle. Tie one end of the twine to the stake, tie the other end to the chalk, and have the second person drag the chalk around. Perfect circle.

Can also use the twine to make happy straight lines, if you pull it taut.
I actually didn't end up working on this; my dad got stuck at work, so I got stuck barbequeing. I think my accomplice ended up roping HIS dad into helping

Reactions today were interesting. Everyone seemed afraid to walk across it XD and a few people recognized "Hey! FMA!" which was cool.
I also printed up a sign to put in the center... since I wasn't there, it didn't get placed yesterday, but I took care of it this morning.
"35 liters of water, 20 kilograms of carbon, 4 liters of ammonia, 1.5 kilograms of lime, 800 grams of phosphorus, 250 grams of salt, 100 grams of saltpeter, 80 grams of sulfur, 7.5 grams of fluorine, 5 grams of iron, 3 grams of silicon, and trace amounts of 15 other elements transmute to forge...
YOUR MOM!!!
--C/O 2006"
Those who will remember, will speak fondly of the warm morning breeze.