This calculation critically depends on a linearity assumption that doesn’t seem at all justified with the large tariff rate changes being thrown about They jus

This calculation critically depends on a linearity assumption that doesn’t seem at all justified with the large tariff rate changes being thrown about They just estimate the derivative of the import volume wrt the tariff rate, then solve a linear equation to set adjust the import rate to equal the export rate. This also assumes the export rate is totally unaffected