Well, it’s an old hot rodders guesstimate route. There’s so much more to this that it’s a bit crazy to write out and explain. Flow numbers aren’t everything and the size & shape of the head/intake ports are another side to the coin. That’s just the induction side of it and only the parts that are moving the air - minus the carb and above it - with the exhaust side and cam/valve train items as well.
At least when you get or shop for a set of heads you can normally find out their air flow numbers. Provided it’s a quality report on a quality port, the next thing you’re looking at is the size of the port. The CSA - Cross Sectional Area
The aspects of a cylinder head are simple but not simple. When you do a deep dive into what’s going on and how things get done and how changes affect everything, you suddenly realize that your neck deep and about to drown in an information overload.
Take a sip at a time to drain the lake.