Surface on line, From a parallel line. Dont know how to start next step

Hey people, i need your help again.

I'am new in grasshopper and i'am fighting every night :D

At the moment i don't know how to start my next step.

I created (for me it is) a huge grasshopper file, not beautifull but it works the most time. 

I need some pillar for my stadium, i created some lines out of my middle point and project it on my roof and my rostrum. So far  It works and i have some surfaces. 

Next step i need to create a surface out of a parallel line (nadir of the rostrum) 

I can't explain it well in english so i try to show it in pictures.

Hope you understand what i mean and someone can help me to start the next step.