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.
https://www.certifiedroadraces.../?type=m&id=1593
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).