Mud construction is a dying art, takes too long, more work than duroc. Both types need a 3/4 subfloor. Duroc or hardbacker will work OK, but you need to follow the instructions for either. They need to be bedded in thinset and screwed down, and any joints taped/sealed. The heat mat can then be laid in thinset.
Of course while all this is going on, you were not paying attention to the toilet flange level. Something to consider before you get too far, since it may need to be changed. It should be level to 1/4" above the finished floor.
Of course while all this is going on, you were not paying attention to the toilet flange level. Something to consider before you get too far, since it may need to be changed. It should be level to 1/4" above the finished floor.