The floor plate is a Rochester, NY, Monroe County thing...it used to be a requirement for installing a toilet. I think they thought all toilets would end up leaking, and rot the floor out, so having a stone plate underneath was a good idea! I ran into that long time ago when I was helping my sister at an older house in the city.
As said, planks, especially those that aren't T&G are not a great support for tile without some help.
If you want to minimize the overall buildup, a 1/2" ply, then something like Ditra would gain you about 1/8" and is easier to install, cut, and carry home than cbu. There are specs and instructions in the 'liberry' on what ply and how to install it.
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Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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