Tax percentage & GDP

Possibly a stupid question: the standard for calculating tax percentages in different countries is to compare total tax revenue to the GDP. But if the GDP includes government spending, doesn’t this distort the result? And if so, are there alternative methods?

Given that taxation is, from a libertarian view, theft, or better robbery, as the use of force is implied, why should a libertarian think about “just” or more “just” taxation systems?

At least I do not care on what arbitrary basis the robbers calculate?

I’m guessing LeV was asking for the purposes of comparing different tax regimes.