Confused Hearth Help: Millboard, Cement Board, Durock, oh my

[BillGriggs]

I'm replacing a hearth and pulled up what (I assume) was an old 1/2" millboard pad with tiles glued to it with adhesive.

I was able to find installation instructions for my original 1980s model and it mentions that the hearth extension should be:

…1/2" thickness of millboard (or equivalent) or 2" thickness of brick to protect combustible floors…height not to exceed 2".​

It was the 80s, so that's all the information provided.

I decided to look up one of their modern installations to see what is currently recommended. This model looked like it had similar requirements for the hearth extension.

A hearth extension projecting a minimum of 20" in front of and a minimum of 12" beyond each side of fireplace opening is required to protect combustible floor construction in front of fireplace. Fabricate a hearth extension using a material which meets the following specifications: a layer of noncombustible, inorganic material having a thermal conductivity of k = .84 BTU IN/FT2 HR °F (or less) at 1" thick. ​

I've got 3/8" porcelain tiles. I just need to figure out what goes underneath it. I've stumbled on several discussions but get rather lost. Can anyone help out?

Note: If important, my subfloor is 2x6 tongue and groove. There is currently an ember plate installed under the front of the fireplace: half under the fireplace, half over the subfloor.