I know nothing about math/engineering, so I apologize if this isn't the proper place to post this question. Maybe I should be in a builder's forum or something. Anyways.... I'm building a round retaining wall on a slope at the corner of my house. I'm using stackable bricks that look like this: The dimensions of the brick are 12" W (at the widest point) x 4" H x 8" D. The backside of the brick is 9" W and it's 8" along the side from front to back. Since I'm building on a slope, I have to step the bricks up 4 bricks high from the lowest point to ground level at the top, kind of like this: http://www.wieserconcrete.com/images/evergreen_3.jpg I want the top row to be 10' ft. in diameter and since the bricks stack with a 1" lip in the back, the bottom row will be 10'3" in diameter. Since the bricks are conforming to a circle of my own dimensions, they will only touch at the widest point, leaving a wedge shaped gap between each brick. Given the size and shape of the bricks, how can I determine the space between the back of each brick to insure that it forms a 10'3" circle?