amount of water for soaker tub

in you case, 500liters (132 gallons) is the full volume of the tub.
You can subtract 15 gallons for the bather and another 15 if you do not want the water to overflow from top.

You can adjust the amount of water as you like.
Recently, in the land of "up to the shoulder soaking" is becoming popular the half-body bathing (keep the water up to the waist).
In this case you would be able to soak for longer periods and enjoy the minus ion atmosphere originated by the steam rising from the tub.