The stone was going in before the steel was drawn.
The primary bath was framed as a single wet room — a tub and a multi-head shower sharing one tiled volume, open to the vanity wall across the floor. The room had large-format marble on the walls and a tiled floor running unbroken from the bathroom into the enclosure.
That sequence set the constraint. Slab and tile were being set while the enclosure was still being drawn, so the steel had to be dimensioned against surfaces that were still changing. D'Hierro measured the opening on site rather than working from the architectural plan.