Nokia Lumia 928
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).