> So we chose to focus on a simple measure of tax burden: national-level income taxes plus mandatory social-insurance contributions as a percentage of gross income

I am not an American, nor that knowledgeable about their tax system, but my impression is that their system has a lot of layers (like state/county/local), which might make it unfair just to compare national level taxes.

Don't forget VAT is double. Property tax is higher. Employment tax (paid by employer, not part of gross pay) is a lot higher.

I always feel like tax should be measured in "if my employer pays 100 extra, how much more can I give another company for a good/service".

I don't know about US but for Germany the answer is between 25 and 40 depending on how low your income is and what your family situation is.

Right, this is a good measure. The national-level one is just proportion of GDP spent by the government, however they collect it, and IIRC the figures are about 25% in the US, vs. about 45-50% in Germany.