The tool would visually represent how much space a person (in a semi-reclined position) of a particular height takes up in the bathtub.
A side view showing a figure in a bathtub - an example is in the attachment to this order.
The user of the tool should be able to determine how tall the figure is in the bathtub. Based on this, the dimensions of the character should be recalculated. The figure must naturally lean against the bathtub. It must not penetrate or levitate.