I want to tile an en-suite and use hardibacker boards on 2 walls where the corner shower is. One wall is a partition wall with wood frame, the other is brick. I have hardibacker 500 12mm and the 250 6mm. I was planning on using 6mm on the partition wall, screwed onto the wood frame and 12mm on the brick wall. Tiles are natural stones 10mm thick. Is 6mm suitable for this or should upgrade to 12mm?