Calculating tube sizes for 4-2-1 Headers

Hey all, I can find plenty of calculators on calculating tube diameters and lengths for 4 into 1 racing headers but I need to build a set of extractors soon and the application requires more down low torque rather then top end.

Does anyone know how to calculate the primary then secondary diameters and lengths or can point me to a good article / calculator?

Thanks

-Clint