Is there a formula or rule of thumb for relating head potential flow to valve lift/ duration?

Other than trial and error , how does one go about matching a cam to the head flow?

 

Nothing simple- it's a very complex system and pro engine builders spend a lifetime learning that kind of stuff.

Now you can do simple rough estimates from basic flow information. Basically you would want to know when, at what RPM, the flow of the head becomes a restriction to getting enough air in and out of the cylinder (whatever sized cylinder that might be). That gives you an idea what RPM range you want your cam to work in to "match" the cylinder head. You don't want a big cam with lousy flowing heads- by the time the cam is up in the range it will make power the head has aready pooped out and you don't make good power anywhere. The other way around (small cam, high flowing heads) is not nearly as bad but still far from optimal.

There are some formulas for flow vs. HP potential floating around out on the web but they are very basic and still won't necessarily give you the answer you're looking for- what will work best in MY engine?

If you want a properly matched (custom) cam you need to consult with a real pro on your choice. The other way is, as you say, trial and error. But anyone whose only teacher is experience is a fool (old saying). My only teacher has been experieince. So you know what that makes me.