I'm trying to figure out if I can drop a tax bracket, from 25% to 15%, using trad 401k contributions. As I've been crunching numbers, I came up with a few questions. While I'm aware tax brackets may soon change, these questions are important to me regardless.

Question 1) What measure of income is used for tax brackets? Taxable?

Question 2) Do I take into consideration the standard deduction or itemized deduction when trying to figure out my tax bracket? For example, using easy numbers, lets say my income after substracting trad 401k and tIRA is 100,000 and my standard deduction is 10,000. When looking into tax bracket ranges, would I use 100,000 or 90,000?