So….more header talk….this had me scratching my head

So is there a "formula" for figuring out what primary tube diameter is best for a specific cam LSA ? I'm sure it's different for max effort race set up and street car. When I had the headers made for my 528 Max Wedge headed solid roller 308R the guy asked me why I wanted 2 1/8 pipes. I couldn't answer and he asked what dyno headers were used. 2 inch. He said then that's what we will make.
Race car only 66 Valiant so no aftermarket headers were available.