I’d say that fair profit is a ratio of materials+labor costs. Basically a supply chain merchant’s VAT. Find a rate at which a well run shop is able to turn a profit allowing it to hire more workers and expand if successful enough, and cap the “fair profit” at whatever that is as a ratio to labor and material costs.

Really the worst hit industries will be ones that are particularly prone to brand taxing, and it actually disincentivizes offshoring since cheaping out on labor and regulatory costs correspondingly limits your upper profit margins.