In SP, the correct question to ask about trade is this:
"Am I spending more CIC than the value of the Factory Output boost that comes from my current trade law?"
If you are spending 30% of your CIC on imports (after deducting the value of exports!) while on Free Trade, that's effectively nerfing your economic growth since you are not getting as much mileage out of the Free Trade boost.
In MP, the equation is very different, because humans, unlike the AI, will organize world trade to be far more efficient.
The question then becomes:
"Am I at the point where I am spending more CIC outside of human controlled allies that are on War Economy or Total Mobilization than the benefit I am getting from the current trade law?"
If you are the US, and you are spending 30 CIC buying human Soviet resources, and the Soviet Union is on war economy, and it's 1937, then even if the US is nerfing its own economic growth some, you are boosting the Soviet Union while they are being pretty damn efficient. In that case, keep the law and benefits of Free Trade. But if you have to spend CIC on some AI minor like Portugal or something, and it's consuming 30 CIC a day, and changing your trade law could fix this, it's probably time to do it. You're throwing CIC away.