About to start my first build with a bent seat tube. Everything seemed fine + dandy until I started poking around the interwebs for some info.

I think I'm confused but maybe I'm not. Would love a bit of clarification.

1. Seat tube angle is 71-degrees
2. Effective seat tube angle is 74.4-degrees
3. My fixture does not have a seat tube offset thingy (a la Cobra's fixture)

My assumption is that the seat tube angle is meaningless and that I should be using the effective seat tube angle to set up my fixture. Is that correct?

The seat tube offset addition is that something necessary (i.e. I should stop right now and fab one up) or just a nice-to-have for builds like this?
How does it simplify the build process?
What is that offset dimension pulled from in bikeCAD?