No the arm length should be "about" double the RADIUS of the circular build plate that is for optimum design.
Think of it this way take the position of the arm where it mounts to the vertical piece and make a line straight down where this line meets the build plate is the end point of the radius measurement
the start of course is the center of the circular build plate. Double this number and that is the arm length. It ends up being the hypotenuse of a right triangle. So the radius is the short side of the triangle the arm length is the hypotenuse and the height is where the hypotenuse intersects the vertical line that gave you the end point of the radius.
This height is also the dead area that you need to add to your desired build area plus the base size to determine the overall height of your printer
Think of it this way take the position of the arm where it mounts to the vertical piece and make a line straight down where this line meets the build plate is the end point of the radius measurement
the start of course is the center of the circular build plate. Double this number and that is the arm length. It ends up being the hypotenuse of a right triangle. So the radius is the short side of the triangle the arm length is the hypotenuse and the height is where the hypotenuse intersects the vertical line that gave you the end point of the radius.
This height is also the dead area that you need to add to your desired build area plus the base size to determine the overall height of your printer