Wow this administration is f**ing batshit insane. I thought the tariffs would be on raw metals, not anything at all that happens to contain them.
The problem isn't creating a reasonable estimate, anyone can do that. Most cheap consumer PCBs are going to be 2-layer FR4 with 1oz/sq. ft. of copper, minus some etched away, with negligible copper in parts like chips. That indeed should get you fairly close.
But there are also 32-layer PCBs, and even PCBs with a solid copper core. And your PCB could be filled with copper inductors! Similarly, it could also be a solid aluminum-core PCB. If I were a malicious customs officer, I would insist that the only valid upper bound is a 100% copper PCB, which is also 100% aluminum, and 100% whatever else. Don't want to pay that? No problem, just provide a certified lab analysis report!
Simple things rapidly get complicated when the goal is to frustrate the process as much as possible. You don't live in a modern economy focused on global trade anymore, you are now living in a Kafka book.