Determining sq cm of a tapered line?

Yeah, I used cad to draw up my design but what I'm wondering that since its a tapered line, where specifically in the line to get them. Like get S1 value here, S2 value there so I can run it through hornresp to see how I did. I apologize if these questions seem silly, I'm pretty new at this type of enclosure building
 
S1 is the cross section at the start of the line (or segment if the taper varies), S2 at the end.
Unless you're using the Offset Driver mode, in which case S1 is still the start of the line, S2 is the section at the point where the driver is centred and S3 is the end.

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