Hello john and welcome to tilers forums.......if you still have some slight deflection in the floor then do not use the hardi backer 250......
Overboard the floor 12mm and use a 2 part flexible adhesive....but seen as you are very new to tiling you would better off using 18mm ply and a single part adhesive that is slow setting , thus giving you a longer time to work with it.....as for the walls then no additional layers will be needed.
Do you know what tiles you are putting on the walls yet...it will be worth thinking about the weight you will be adding to the walls if choosing large format tiles......
If it makes things easier ..have a look at what you want to put on the walls and floor then let us know and we can advise you what materials to use......
