Flooring Tile - sub floor for new tile floor in bathroom

What is the thinnest subfloor I can use under 12"x12" marble tiles.
presently I have a 1x8 t&g pine subfloor over joists 16"oc.
If I add 3/4" on top of that to be flush with the rest of the finished floor. Anything more will sit higher.

The tiles are 3/8" thick, so if I were to use 1/4" hardiboard or similar, I figure that is the closest I can get to matching the rest of the finished floor with my tile and thinset.
Oh yeah, I'm also adding Nuheat mats under the floor, so figure they add an additional 1/4".

Is the 1/4" hardiboard too risky?

Your entire plan is too risky at this point.:)

When installing stone tile the substrate requires twice the strength of that that is required for ceramic tile.

Your first concern cannot be the transitions. Your first concern is the needs of the stone, those requirements must be honored above all else. The transitions are secondary and can be dealt with but that issue cannot be allowed to dictate the technique of the stone installation.:)

The 1X8 subfloor is a serious issue and cannot simply be covered with Hardibacker no matter how thick you use.:)

Thanks Bud,
I appreciate the feedback. What would you recommend?

I'm not sure if this matters, but the place I bought the tile from has wear codes assigned to all the tiles.
They recommended not choosing a tile for the floor that is less than wear code 5. The tile I have is wear code 8. (The scale is up to 10). I thought I was buying a stronger tile by getting this one.
I rechecked the label. it is "Honed Egyptian Marble ECM"

I appreciate your help.