If I'm understanding correctly, marginal tax rate is calculated by subtracting the standard deduction (since I'm not itemizing), as well as other deductions, such as child and education credits, correct?
Sort of, but not really.
Marginal tax rate is defined as (change in tax) divided by (change in income or contribution).
Your taxable income is your gross income minus pre-tax deductions (401k, insurance, FSA, deductible tIRA, etc.) minus [standard or itemized] deduction.
Your tax is calculated from the taxable income.
What you owe is the tax minus any credits.
Have you tried using the spreadsheet suggested earlier?