Subfloor build up advice for tile

[Hammer]

If starting from joists, what subfloor would you use these days? 18mm Ply first and then a 6mm backer board layer? (correct thicknesses?). Plan is to tile floor with porcelain tile 20x20cm, electric heating mat under. Thanks.

 

[AP tiler]

18mm chipboard glued and screwed, 6mm wedi glue and screwed with washers, then vario pro/ ditra heat type matting and tile. Will be a nice height for threshold to meet carpet