The estimated tax penalty is variable and approximates 4%, but only for the period of the underpayment. For example, the penalty on $1,000 paid 30 days late would be 4% x $1,000 x 30/365 or $3.28.
Could you add a link to the IRS.gov page that explains what late fee to pay?
<a rel="nofollow" target="_blank" href="https://www.irs.gov/taxtopics/tc306.html">https://www.irs.gov/taxtopics/tc306.html</a> which then links to the (more confusing than usual for tax forms) Form 2210.
How do we know if we are late? Each quarter they are checking the witholding? What does it work if it's joint filling with another W2?