quote; When they say "thick wall", they mean a wall that is already finished with tile, stone, etc. But in either scenario, the plastic disk has to be behind the inside of the wall.
That is almost completely wrong. With a "thin wall which means the side of a modular tub or shower, there is enough latitude that the plastic ring CAN be behind it with the wall clamped between the disk and the trim ring, but that is COMPLETELY DIFFERENT than a "thick wall where the ring MUST BE even with the finish wall, NOT "behind the inside of the wall". IF you do it that way you can be almost positive the valve will be installed to deeply to finish it without an extension kit.
WE were installing the plumbing in a house for Jack Ruby's brother. It had 10' deep basement walls. In order to keep the ground from freezing they threw straw into the basement, then it rained, and finally it froze. Then they installed the first floor deck over the basement which kept the sun from shining down there. It was still frozen 3 months later, and it was like trying to chop plywood because the straw kept the frozen ground from "breaking up" when we used a jack hammer. WE also had to get surplus army "Arctic boots" to keep our feet from freezing, and even then we could only work so long before we had to come out and warm our feet up. He also installed a "hot pitch" roof, which was a lot hotter than tar, so the roofers threatened to go on strike.