the optimal depth of an ofuro

Hi, I have been admiring the products in your website.
I have one question:
-When I measure my partner’s body, from her seat to just-above shoulder height measures 24″. Including a small seat in the tub (say, 8″H), brings the interior dimension of the tub to 32″H. It seems that in order for her to soak without slouching her posture, the depth of the water would have to be at least 32″H. Therefore I imagine the exterior measurement (including feet or supporting slats) would bring the tub to around 34″H. Yet the deepest (standard) tub I see in your listings measures 690mm H, which is about 27 inches.
Can you please explain why the depth is as you have listed, and not greater, per the above situation. My partner is about 5′-6″, so she is about average height.
Thank you.
I will look forward to your reply.

Usually, the full immersion “up to the shoulders” bath is done sitting on the bottom of the tub (so to say, without seat)
Actually traditional japanese bathtubs do not have a seat.
The seat is used for a “half body bath”, for reading while in the tub or to get a pause from the full depth bath (when you feel too hot)
To have the full bath, you can sit in the tub on the opposite side of the seat . Alternatively, you can remove the seat plank.

Also consider that when you relax in the tub, rather than keeping a 90 degrees sitting posture, it is easier to slide a little forward until the feet touch the other side. (This is also why the tub should be not too long) In this case the shoulder line lowers a few inches.

We can make extra deep tubs (some examples in the custom tubs section), but the disadvantage are:
– increased specs to contain the extra water pressure (and therefore increased cost)
– you will need some steps to climb to enter and egress easily (or recess the tub in the floor)
– you will need more hot water each time.