OP
Dumbo
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the tiles on the concrete need https://www.tilersforums.com/forums/tile-adhesive/ say 3mm so that negates bringing that into the equation . So what the op needs to allow for is the difference in floor heights prior to tile i.e. 6mm Hardie plus https://www.tilersforums.com/forums/tile-adhesive/ for Hardie which is not much ( especially for me as I tread mine in )