Thank you, hj!
I'm glad I wrote about what I was told, because it's been my experience that the product sales "experts" rarely know anything about the products they sell.
OK, with that knowledge, I looked at the installation guide again, and it looks like they want the plastic guard ring (which is kind of clipped onto the permanent ring) flush with the finished wall. By finished wall, do they mean the edge of the tile or stone I will finish it with? Or do they mean the edge of the durock?
If it means the tile/stone edge, then I would account for what will go on top of the durock: thinset, kerdi, thinset and 1/4" tile. How much should that be all together- a little less than 1/2"?
I'm attaching a shot of the "thick wall installation" section.
Thank goodness for you guys.