Why is ther no formula to figure out port volume?

The bench need a pitot tube to get airspeeds in the localized areas of the port. Volume of the entire port tells little when you get really into things. It's volume of small sections of the port that combine to create fast and smooth. That's where the tueb and local speeds come into play. I think something like a Super Flow 600 is the lowest level bench that can use the pitot tube, at least with factory software. Speed is a big part, but not the fastest outright speed. The fastest non-turbulent flow is what you want. Too fast is when turbulence occurs due to speed, and the result is what's called port choke.