This about as good a picture as I can take without removing the radiator. Looking straight down, you can see the tensioner body and just ahead of it you can make out the hex of the 19mm head bolt. If you view from this angle, you can guide a long 19mm socket into place. The view might not be as clear with the belt in place.
The belt is off the pulleys in this shot.
You can also see the pointer, triangle and line on the tensioner body. The pointer is about at 70% of proper tension in this shot. I'm not sure where proper tension is; either the end of the triangle, the gap or within the line. You can set it pretty precisely using the hollow nut.