Sulpt command to have a new solid

[Anonymous]

task: 

getting a new solid using sculpt command between a plane/surface and the existing solid (for example to obtain a new solid to calculate the inner volume of a tank) .

actually when I use the sculpt command and select a plane (level of liquid) and the existing solid I can't select new solid in the options

what I can get is only the solid plus the volume between the solid itself and the plane.

the work around I use is to copy the solid as a surface than delete a face. in this case the sculpt between the plane and the surface will let me select the new solid option.

Regards

Diego