Marginal Vs Effective
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The marginal rate is the tax rate applied to your next (or last) dollar of income, based on which tax bracket that dollar falls into, while the effective rate is your total tax paid divided by your total income — an average across all brackets your income touched.

When deciding whether extra income (like a bonus, overtime, or an investment gain) is worth pursuing, or when evaluating a tax deduction's real value, the marginal rate is what actually applies to that additional income or deduction, not your lower effective rate.

Because a progressive tax system taxes lower portions of income at lower rates, and only the top slice of income is taxed at your marginal rate, blending all the rates together into an average (the effective rate) will always be less than or equal to that top rate.

No, this is a common misconception — moving into a higher bracket only means the additional income within that new bracket is taxed at the higher rate; income you already earned in the lower brackets stays taxed at those lower rates, exactly as before.

Use your marginal rate, since that's the rate that will apply to the additional income from the raise — the effective rate reflects your average tax burden across all your income and does not represent what happens to the next dollar you earn.