Awesome, sounds like you are on the right track.
The withholding calculator estimates your taxes (https://apps.irs.gov/app/tax-withholding-estimator). It is not 100% accurate, but it gives a great idea on how much tax should be withheld. It is important to answer all the questions correctly, in order to get the closest estimate.
Here is some helpful info from the IRS website regarding the penalties:
You may avoid the Underpayment of Estimated Tax by Individuals Penalty if:
- Your filed tax return shows you owe less than $1,000 or
- You paid at least 90% of the tax shown on the return for the taxable year or 100% of the tax shown on the return for the prior year, whichever amount is less.
And of course, the tax must be paid before the filing due date.
