Sweet spot is immediately under the rear camera lens. Ring must be oriented such that it is perpendicular to the silver bar (i.e., makes a cross with it).
Have not yet tested with a case since I do not have one, however it does read from ~1cm away through the air, and it reads through a thin layer of cardboard (at the very least) as well.
I should also mention that this test was with the V1NTAGE ring, not the alpha.
This phone is very similar to the Lumia 920. The sweet spot is in the middle of the silver strip on the rear of the device, right on the bar that separates "Carl Zeiss" and "Pure View". Orient the ring perpendicular to the silver strip (parallel with the silver bar) to get a clean read.
I was unable to test with a case, but can get a clean read from about 1 cm away using the V1NTAGE ring (not the alpha).