I don't understand why GDP is part of this calculation. It can be done more directly with (taxes/persons). Also, although greater taxes may depress GDP somewhat

I don't understand why GDP is part of this calculation. It can be done more directly with (taxes/persons). Also, although greater taxes may depress GDP somewhat, there are other factors at play that makes the U.S. the home of the most productive workers. I still think the more interesting metric is tax burden as a proportion of total income, not absolute taxes paid per person.