You can pace out a circle using bladelengths or steps. Mark the center of your eight and measure eight to ten bladelengths or about four paces. Make a mark. This is the center of your circle. Measure eight to ten bladelengths or about for paces further along the long axis and make a mark. Measure eight to ten bladelengths or about four paces to either side of the center and make more marks. Now you have four marks on your circle: one at the center, one at the top, and one at each side. Connect the dots to make a circle. Repeat for the other side.
If you want to be very accurate, calculate the ideal radius of your circle in inches (1.5 times your height) and measure the length of your blade. Divide the radius by the length of your blade to get the number of bladelengths to use.