Calculate Estimated Tax on Hobby Business LLC
You are on the right track, for tax purposes its all ordinary income at the end of 2016. If the free lance "employer" will withhold fed,state and local tax, then that takes care of your estimated tax. If they can't or won't, you will need to make those estimates and make payments quarterly for the fed and state tax at your projected tax liability.  Or, you can bump up withholding by your day job employer and cover your expected tax liability at year end without making estimated tax payments.