The density radius is not from a main entry or from the center of the project site. It's based upon an offset from the LEED project boundary. Because of this, it will likely end up being a weird shape, rather than a typical circle. Also note that measurement to basic services are based upon walking distance rather than a radius.
Where do we measure the radius from? In LEED 2009, we measured it from the main entry, but the LEED v4 language is different.