Best placement in my experiences is either on the upper back behind your non-firing shoulder...but I'd recommend on the front of your non-firing shoulder.
Lower back placement will cause interference and can damage the antenna while moving, especially if you have a pack or reservoir.
My personal preference is on my back as it allows for me to fit more pouches on the front and there's nothing to snag my replica on.
But in all reality, there is no set standard or "right way" to place a radio. If you look around at pictures from various nation's, you'll see everyone does it differently.
Some put it on their back behind the shoulder, some put it in the center of their chest, some put it on their pack's webbing, etc...
It's really a matter of personal preference and what works best for you and youre gear set.