Diagrid from revit levels (parametric monkey tutorial)

I've been following this tutorial on how to create a Diagrid from revit floors and levels. (https://parametricmonkey.com/2016/06/29/structural-diagrid-via-flux/)

I'm currently at step 4, and I'm stuck because although it generated the diagrid, it looks messed up and it isn't aligned with the imported revit floors. I've tried looking into the 'Structural tubes' cluster inside the GH definition however, it's certainly too advanced for me.

Could someone explain how the input parameters work? (and maybe how the convoluted cluster works). I also don't get why the ring beams are oval-shaped when all the diameters are set to 5000.

The diagrid I'm trying to create is just a simple circle with the same diameters at every level. They're also slightly offset from the edge of the slab.

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Or maybe is there is there a way to create the diagrid independently of the revit floors? (e.g. based on a cylinder).

Thanks.