you are correct that the IRS standard sales tax deduction is not AGI times the general sales tax rate. for example I used single, 100,000 of income living in Chicago, IL, the IRS website gave me a figure of $1,333
for Beverly hills,Ca same facts the number was $1,242 for New York City it was $1,217
so it seems that the IRS concludes you spend little over 10% of your income on taxable purchases.
QCDs wouldn't count because none of it is spent on taxable purchases.
