Why is ther no formula to figure out port volume?

I would say that this is the preference of the builder to use what he feels he needs for valves. Most put them in because someone else did, not because they know how it effects the port flow. If the 2.02 heads were so efficient Mopar would have kept using them and in 1972 they used 1.88 intake valves in place of the 2.02's, partially because of the emissions. But they also found that the 1.88 valves were more efficient. And this is what they stayed with until the end of the LA engine.

In fact even the modular engine of today doesn't use a intake valve larger than 1.925 with a 2.00 valve as a only replacement, not to say that they need them but they do make them. But once again this is the end users decision as to what they want or need to use.

From everything that I've flowed of the 302 heads with a 1.88 and with the 1.78 intake valves the larger valve almost never made more average flow than the smaller valve, and they both made similar peak airflow numbers. But when used on the engine the smaller valve heads were 2 tenths faster. But hey what do I know.