What you say is true. But if the RD is going to set up a semi-circle TA, you might as well tell him how to do it without adding distance to the course. Below is an example from one of my maps.
It's really not even necessary to provide a diagram of the semi-circle. You can just say in your description where a single-cone TA should be placed, and where the center of a semi-circle TA should be placed (like I did in the mile mark descriptions).