That should work...have you checked the stiffness of the subflooring to ensure it is strong enough for tile? You shouldn't just assume it is. Normally, 3/4" plywood is near the minimum, and without knowing the depth of the joists, their spacing, and the longest unsupported length, you should not assume it is. It doesn't matter how big or small the room is, it is the design of the structure under it, on the entire floor that is important. If you check out www.johnbridge.com on the top of the forum's menu bar is a link to "deflecto" which will allow you to check, assuming you know the makeup of the floor structure. You need L/360 MINIMUM for ceramic, and L/720 for stone tile. Note, Hardi backer (and any brand of cbu) is NOT considered structural, so on a floor, 1/4" is fine. Tiling questions will get alot more action on John's site.