What size Schluter Schiene should I use with 18 .in X 18 .in porcelain tile, since it uses a thicker layer of thinset? 3/8 .in, 7/16 .in, 1/2 .in?

Order the Scheine trim approximately 1/16th - 1/8 .in taller than the thickness of the tile you are using to account for the thickness of the thin-set. If the bed of thin-set is on the thicker side, we would suggest measuring the exact thickness of your tile, and adding 1/8th inch, as that should be appropriate for a bed of thin-set for an average 18 x 18 tile.