I am trying to extract the lower half of a circle using the constructdomain2 command.

The context of this is that I am modelling a stream in a topographic model. The stream bed has been modelled as a negative of a pipe-variable (topographic model with pipe subtracted from it). I am now looking to ‘fill’ this with water which is to be modelled by the same pipe, but with the waterline just below the edges of the stream (using Isotrim command to remove the top half of the pipe).

Thus, I need say, the bottom two thirds of the pipe. This is well explained in this video: https://www.youtube.com/watch?v=t8ixuJ2t9q8&ab_channel=RhinoGrasshopper

It says that a domain of 0 - 0.5 will extract the bottom half of the circle. I would like a subsurface of approx 0 - 0.4 (so that the water line sits below the edges of the stream). Inputting these numbers as a domain works except that I can’t control where on the circular section of the pipe this domain starts.

How do I control where the domain starts on the circle?

Thank you.