Would appreciate you (or anybody else) shed some light on the economics of the thing.
But basing the tariffs on import/export ratio makes sense if your goal is to be a net exporter with every country, as it discourages imports until that's the case. It's still somewhat arbitrary though; my guess is that the White House is pursuing that goal mostly for political, not economical reasons.
Given that both elasticities were set to cancel each other, that's why you get a flat trade deficit/imports calculation.
This is, sadly, the way a freshman econ student would calculate tariffs.