I don't calculate my tip on the full total. I always calculate the tip pretax. So $100 bill $10 tax. My tip $20, not $22.I go 20% unless we hog the table by drinking longer then I tip more. But dang, I really struggle on paying 20% tip on sales tax. I begrudgingly do it, but dont like it. It seems like its tipping a tax.
If it is a small bill like breakfast I tend to be even more generous than the 20% but still based on the pretax number. I don't see paying a tip on the tax either.