Apply LookAt change to position

I had a text which is a overlay text on the object.
I had applied LookAt to make it rotate face to the camera.
But because it's a overlay text on object so it should also RotateAround the object.
I don't know how should I calculate the angle to make it work like LookAt but changing the position instead of rotation.
Thank you.
Below is a drawing in case my describe is not clear.