most don't test today, they rely on what their competitors with a similar combo run, what they've read on an internet forum, evident by this thread
and/or what companies like Burns tell them they need and even then experts aren't always right.
I remember a lengthly conversation I had one night with Jere Stahl who refused to believe I ran as quick as I did with 1 3/4" primary tubes on my 700HP 468.
The fact remains that most run headers that are not one but two pipe sizes too big.