Subtract your elevations then > insert story below with that result in the "height to next" box.
The AC usage for elevation heights is that you should input your project elevation in "Project Preferences" in the Options Menu. As you will notice you can only input elevations directly in stories for the highest story. No story that is below another can receive elevation input directly. Hence the subtraction required to get a "height to next", when inserting new stories.
BTW - Instead of placing the site survey on a story, place it in a worksheet and then place the worksheet on the layout. If you need the boundary lines, setbacks etc., then redraw those in the first/main story. You can use trace to see the survey on any floor as work. I think you will find in the long run that keeping imported 2D information in worksheets will help you find order and consistency between projects.
