japanese bathubs are used for soaking - with the water to the shoulders. But how much should an ofuro be deep?

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.