Circular Stair Layout.
Snap a construction line that follows the top of the stairs. Mark off the center point on the construction line using the inside radius dimension. Mark off the Dimension X and Dimension Y construction lines. These lines are at 90° to each other. Snap a line through Point X,Y for the Stair Rotation Angle. Draw the inside and outside radius arc's, using a trammel. (stick with a nail at one end and a pencil at the other end.)
Mark off the Tread Arc Length on the inside radius curve to locate easch stair tread.
Snap lines from the Radius Center Point through each of the Tread Arc Length Marks on the inside radius curve. These snap lines will locate the treads for the circular stairs.