Calculate how much the gross income is for each contribution rate and then
calculate the tax (0.15 x Gross) on the gross income and compare to what you
would pay with no contribution.

Tax = 45,000 * 1.0 * 0.15 (No 401K contributioN)
Tax = 45,000 * 0.99 * 0.15 (1% 401K contribution)
Tax = 45,000 * 0.98 * 0.15 (2% contribution)
etc.