I'm refurbing my bathroom after a leak. I've ripped up the old tiles and the 6mm ply they were sat on and the floor boards underneath are in a bad state. They are 20mm t&g and I was wondering if I can replace them with 18mm t&g ply boards. The wife has decided that she wants UFH so I'm trying to keep the make up thickness to a minimum. After putting down the 18mm t&g ply I was then going to fit 6mm insulated boards, then UFH mat, SLC and then tile on top. My joist centers are 400mm, does this sound ok ?.