How to create a Hz surface exactly on the edges of a Brep (for trimming)

Hello everyone,

Attached is an algorithm I'm working on, I want to know if it's possible to create a horizontal surface directly under the created geometry so I can split the geometry with it, in the attached file I used an external surface drawn on Rhino but of course if the geometry gets larger than the plan every thing is screwed up..

If there's another way for trimming a brep that would work to, I tried converting it to a mesh and then splitting by an infinite plan but it didn't work..

Thank you..