So, for the purposes of calculating my Fed annual taxable income and tax liability can I assume:
Salary - Std deduction - (Exemptions * #of exemptions) - Pre Tax Deductions (401k/HSA)
e.g. Single with 2 additional exemptions means I would only be taxed on:
$100,000 - $6350 - ($4050 *2) - $18000 = $67550
With a total liability of $12,788 and an effective tax rate of ~13%
That's a way to come up with a rough estimate. Alternatively, you could look at your 2016 tax return to see what your taxable income was then. Your portion of medical insurance is also a pre-tax deduction. I'm not sure how you calculated your number though.
With 2 additional exemptions, I'm guessing you file as head of household vs. single. At $67,550 in taxable income you are at the 25% marginal tax rate (meaning each additional dollar earned is taxed at 25%). Your tax liability would be $11,185 if you contributed $18k to your 401k. Whereas, if you contribute nothing to your 401k, you would have $85,550 in taxable income and your tax liability would be $15,685. Therefore, you would realize a tax savings of $4,500 when comparing contributing $0 vs. $18k to your 401k. Meaning, you only need to come up with $13,500 of take-home pay to put $18,000 into your 401k.
I don't care about my effective tax rate personally. It's all about comparing your marginal tax rate now vs. your marginal tax rate in retirement. And your marginal tax rate in retirement starts at 0% until you build up enough tax-deferred assets that will generate enough ordinary income to move you up marginal tax brackets. In order to provide a certain dollar amount each year, you need roughly 25x the amount in investments. Said another way, your investments should produce 4% annually. If you have Roth IRA assets, that's a portion of your income in retirement that will not be taxable. Further, social security income is not always taxable either. At most, 85% is taxable, but many people may be taxed on less social security income depending on their income.
With all that said, in your situation, the best bet would be to put the max into your tax-deferred 401k and HSA as well as your Roth IRA. That way you have some tax diversification as tax rates change.
None of this considers the ability to do catch-up contributions.