Alan,
You could try to fit a cylinder to the tank and clip the way you suggest,
but that would be a little bit involved and may not be accurate of the tank
is distorted significantly.
You can potentially used a signed distance function from the tank surface
to compute what is inside vs. outside.
I have a user that wants to see how quickly water flows out of a commercial
water tank, if it has a seam split. If the tank doesn't change diameter, this
is easy - how much water is outside of a cylinder clip. But, if the tank is
expanding (due to damage), is there a way to have the radius of
